I relied so heavily on tips from other travelers on my adventure through New Zealand, so I'm sharing lists of things I learned on my 10-day roadtrip through the South Island. If you're planning a trip, I hope this will be helpful! You're going to have a wonderful time!

Click here for my main trip report with tons of photos.

Airlines

Preferred Seating (not Premium Economy) on Air New Zealand really paid off ($50 from LAX --> AKL). I sat in one of the three rows in the back with only two seats, and it was a great experience.
I definitely preferred Air New Zealand over Virgin Australia. ANZ offers 33" seating on their 777-300s, and that extra two inches is everything over the course of 13+ hours. Their service is also amazing, and I can't say enough good things about my long-haul experience (LAX-->AKL). The short-haul (AKL-->CHC) was pretty standard and a bit tight, but nothing horrible (sit on the right side of the plane for an extra inch of legroom, I think up until row 21).
This page has all of Air New Zealand's current air specials. Look for some killer deals here (I have it bookmarked), like $900 RT from LAX.
Air New Zealand provided a fleece blanket and a pillow, which was awesome due to the extreme cold in the middle of the flight. I mean, teeth-chattering cold. Dude. I recommend bundling up.
I hate to add health information, because I may have just lucked out, but drinking three liters of water per day and taking extra vitamins for the week before my trip seems to have staved off illness. I get sick a lot after airplane flights and was in NO MOOD to deal with it this trip. Who knows, my inner evil eye may have made the germs flee more than the extra Vitamin C.

Baggage

Photographers! This is a big deal! Air New Zealand and Virgin Australia have very strict weight policies for carry-on bags, unlike U.S. airlines. Your larger carry-on can only weigh 7kg (just over 15lbs!). This is really, really tight for camera gear, especially since my rolling ThinkTank International bag weighs a cool 10lbs (I ditched it in favor of my Vanguard backpack, which weighs 3lbs.). I put my batteries in my pocket and put one of my heavy lenses in my smaller handbag.
I was carrying flashes and Pocket Wizards in my checked bags (for the wedding), and I stripped all the batteries out of them for weight.
While my carry-on bags were not weighed, I've heard that they will sometimes weigh bags both at check-in and the gate, and if staff is moving your bag around in the overhead bin and notices it's too heavy. A photographer friend had her camera bag taken from her and checked because it was too heavy, so it does happen.
Air New Zealand allows you to check extra bags for $1 on most flights. Cool.

Driving

Driving on the left side of the road was hard the first day, then slowly became easier and easier. The hardest part was any kind of turn, which felt incredibly weird. Take your time, drive slowly, and if you're feeling sketched-out, pull over and breathe for a second.
I read the New Zealand driver's manual and a bunch of blog posts before I left the States. It was incredibly helpful, especially the sign recognition pages. Having that information sloshing around in the back of my brain definitely made the first day less stressful. Plus really, are you going to sit in the lot and read it before you start driving? No way.
Cars in New Zealand are pretty much dedicated to roadtripping, and even the nicer ones are often not new. That being said, be prepared for less-than-perfectly-modern amenities, as I note below.
I rented from Abell, which is off-site at CHC. They were totally fine and my car didn't have any major issues. There was a minor panic because my flight left at 9am from CHC to Melbourne and they only open at 8. The owner was great and took us himself (he seems to do this regularly, bless him), but I should have caught the timing.
The car's USB port for the audio player didn't work very well, so I was happy I had a standard audio cable. I was also happy I'd brought my little dual-port car charger so my phone and iPod were always charged up.
Your US insurance very likely doesn't cover an international car. I opted for the daily "excess coverage" from Abell, which covers the roughly $4,000 "excess" when you have an accident (the rental fee usually includes everything else). Considering a glacier taxi bus backed into my car in a parking lot, I was very grateful I had it. There's still a deductible ($200 NZD), but it's negligible compared to the alternative.
I didn't need the international drivers license I purchased from AAA, despite that being listed as a requirement from my rental car company. It's probably worth calling ahead.

Entrance / Visas / Money

New Zealand does not require US citizens to get a visa in advance--you get it at the airport upon arrival. We are a "Visa Waiver Country", so vacations are easy. Sweet as!
Australia does require a visa before arrival. Apply online in advance, pay roughly $20, and you're set at the airport. It attaches to your passport number, and there's no headache--it's all automated.
If you have an e-passport (there's a bowtie-ish symbol on the front of your passport--you probably have one!), you can go through the quick lane at the airports, called the SmartGate. You don't even have to talk to a human. It's so fast.
NZ has a bunch of "you must declare"s on their incoming passenger form, including shoes used outdoors, camping equipment, and food. They weren't concerned about my hiking boots, asking if there was any soil attached (my boots were clean). There's more about all this here: http://www.customs.govt.nz/inprivate/onyourarrival/quarantine/Pages/default.aspx
I flew PDX -> LAX -> Auckland -> Christchurch. I checked my bag in LA with Air New Zealand, then had to re-claim it in Auckland. The bag went through an extra bio screening as part of customs, then I dragged it over to the domestic terminal and re-checked it before boarding my flight to Christchurch. On the way back, I had to do something similar, where I claimed my bag in LA, then re-checked it once I cleared customs.
I converted some cash before I left so I had about $1,000 NZD with me, mainly because it was listed as an entry requirement (which was never checked in the airport). It was nice for gifts and food and the odd hotel where they charged extra for credit cards.

Internet / Phone

Internet is available all over the place, and is almost always available at the hotels. The quality and speed is almost universally crappy (Facetime didn't work anywhere). You are often handed a voucher or card with a limit of 500MB-5GB, depending on where you're staying or eating. It's usually enough for email, iMessaging and a few image uploads at least. The Milford Sound Lodge gave me a 100MB voucher per day, which goes away immediately if you don't disable all the auto-checks on your device.
I opted for the Global Passport through AT&T for my phone, and used it for text messaging, phone calls home, and data for the few times I got lost and needed Google Maps. Coverage in the outer regions is spotty, but it's decent most places inland. In my opinion, it was well-worth the extra money to feel safe and be able to call home regularly.

Time Zone - Cool Fact

During Daylight Savings Time, New Zealand is only three hours off Pacific Standard Time (and a day ahead). Otherwise, it's five. I was so pleased at the complete lack of jet lag. SCORE.

Insurance

I added international coverage to my business insurance before I left, so my equipment was covered on my travels. When you're toting around about $20k of equipment, it's the only smart thing to do.

Sun

The sun is SO direct in New Zealand, and there's a 10-minutes-to-sunburn time (they have the highest rate of skin cancer in the world). It's oppressive. Be prepared.
Sunblock is effective, but only when applied early, often and well. Driving was hard on my hands, even with a lot of application of sunscreen, and even on cloudy days. I definitely got more of a tan than I wanted.
Bring a sunhat. Also, lightweight, long-sleeve shirts are awesome so you can cover your arms on warm days.

Photography Equipment

I brought a pure plethora of lenses: 14-24, 24-70, 50 Art (Sigma), 70-200, and a manual-focus 50. I used my 50 Art and 70-200 lenses more than any of the others by 10-fold. I used my 14-24 only a handful of times, and could (shockingly) have left its damn heavy self (along with the bulky WonderPana polarizer) home. I also would have left the manual 50 behind, even though it's pretty.
I used my polarizer a lot. Like, most of the time. I would have been severely crippled without it, at least to get the shots I wanted. There's just so much sky and water. Having all of my lenses except the 14-24 on the same thread size (77) made it easy to just bring one polarizer. I did use my NDs, though not a lot. I still was happy I brought them.
I brought two camera bodies. I was really glad I had two anyway, since I lost my balance and banged my knee into a rock on Motukiekie Beach and dropped my camera into a puddle. The camera still worked, but I torqued the lens mount, leaving me unable to put lenses on after I took off the 50mm I'd been shooting with. I would have been devastated without the second body. I would take two again. Oh! And other cool thing? Since driving in New Zealand involves stopping a LOT, having two cameras with two lenses in the back seat made for a lot less lens-swapping(/sensor dirtying).
I also brought my little Lumix, which I used for hiking, kayaking, and general fun shots of Anne and I doing stuff. It also has killer macro, so I could leave my macro lens home. And video! Yay! I LOVE that camera, and it goes everywhere with me.

Gadgets / Bags / Stuff I Love

Lightweight luggage scale -- with the militant limits on baggage weight, not having to guess was awesome
Turtl pillow
Eagle Creek 28" FlatBed Duffel
Ellington Hobo Bag
Packing Cubes
Vanguard Sedona Camera Backpack (super lightweight)
Goodbye Sandfly spray

Glad I Packed

Eyemask
Ibuprofin
Bandaids
Audio cable for the car

Super-Helpful Websites

SeatGuru - I use this extensively to figure out where to sit on airlines I don't travel regularly
Booking.com - Booking for most of my hotels, easy organization, easy conversion from USD/NZD
Milford Sound Lodge - Booking + activities
Tripadvisor - Reviews
Kayak.com - Flight research
Google Flights - Flight research
Google Maps - Surprisingly-accurate time tables for driving + boutique hotel listings for middle-o'-nowhere

Places I Stayed (all highly recommended)

Cool Places & Stuff

Roscoe's Kayaks - Milford Sound
Mitre Peak Cruises - Milford Sound
Mrs. Woolly's General Store - Glenorchy
Glenorchy Hotel Pub (amazing dinner)
Mountain Jade Store - Hokitika
Hermitage Hotel (fantastic dinner buffet at Mt. Cook)