Rain boots are critical and I mean critical! Get a pair you like and that fit well. Despite being much more comfortable hiking boots don't have much of a lifespan in the tropics. Insoles are a great idea but get the kind that are gel - the other kind tends to soak up water.
Clothing is very much down to personal preference but I would err on the side of caution. Some people are fine wearing short sleeve shirts but I like having as much skin covered as possible. Nylon hiking clothes are great for staying (pseudo)dry but they are expensive. Personally I'm willing to splurge for field pants but not shirts since you're far more likely to get muddy/wet on your lower half. I like the kind with tons of pockets. I am super cheap when it comes to socks - I buy bulk cotton socks and then ditch them after each field season. Good rain gear is important but stick to ponchos. Rain coats are great but you can't cover your backpack and you have to take your pack on/off every time you want to don/doff it. Take a spare - branches tend to shred ponchos.
Generally it is a good idea to minimize the amount of stuff you carry with you so aim for a fairly small backpack. I like one's with waist straps otherwise I'm prone to headaches from the strain on my shoulders. Even light packs take their toll after 12 hours. Some people like to carry fanny packs or vests with lots of pockets to store other stuff. I'm going to try out the fanny pack idea this coming field season - I can see the appeal of having a few items within reach without having to take off your pack.
Water will be one of the most important (and heaviest) items you have to carry. Some people prefer water bottles but I think water bladders with drinking tubes are hard to beat for convenience. Being able to drink without having to stop and get a bottle out of your bag is great if your study animals refuse to take a break with you. Whatever system you end up choosing make sure you carry enough water for the entire day, for most people this is around 2 liters.
Take a decent first aid kit with you. This should include band aids, gauze, bandages, alcohol swabs, neosporin, ibuprofen, benadryl and tweezers. If you are allergic to bees or wasps make sure to bring an EpiPen. You should also bring down anti-diuretics in case of stomach issues. You might consider anti-itch cream; I gave up after realizing I had more bitten than unbitten skin left during my last trip (you too can relive your childhood experiences with chickenpox through the magic of chiggers). On that note - you'll have to find out what kind of bug spray and how much works for you. I carry pretty strong DEET-based spray but I only break it out when the mosquitoes are really bad. The rest of the time I try to get by with natural bug spray. You can buy permethrin to treat your clothing but you do become the grim reaper for any insects that land on you, including the awesome beautiful ones.
A good set of binoculars is probably the most important item for primate fieldwork. Woolly monkeys like to hang out really high in the trees so they are pretty much impossible to see clearly without binoculars. I really like my Nikon Monarch 10x42 binoculars. You definitely want a pair that will let in enough light so avoid going too small. I am also a fan of binocular harnesses in place of the typical neck strap - they distribute the weight over your back. Believe me your binoculars may not seem heavy now but they will by the end of the day. You'll also need a waterproof watch that you can program to beep at intervals.
Headlamps are a must for fieldwork. We enter and leave the forest in the dark so getting caught without a headlamp is downright dangerous. You'll definitely want a spare or two. Make sure to carry backup batteries as well.
Finally, a note on being female and in the field. Having your period in the field sucks so make sure to bring enough supplies. Make sure you keep tampons/pads dry otherwise you may find yourself running out sooner than you'd think. Store them in ziploc bags and preferably somewhere dry. There is nothing more depressing than reaching for a tampon that's decided it wants to be a magic growing capsule (and you don't even end up with an awesome dinosaur toy at the end). Bras are another hassle; most people go with sports bras but I think if you don't need the support just ditch them altogether; layers + humidity + heat = cranky field researchers. If you have long hair you'll definitely want to keep it pulled back in a braid or ponytail. You might want to consider getting a haircut beforehand. At our site the showers are cold so anything that minimizes the time it takes to get clean is a plus.