Rubber Sandals - Canvas or leather sandals are great but take a long time to dry tend to collect sand. A cheap pair of $5 Wal-Mart sandals will be easy to wash when putting your feet under a water spicket and won't get ruined walking in the water and along the beach.
Suntan Lotion - Obvious, the sun in Jamaica is strong and you spend all your waking hours outdoors so suntan lotion is a must.
Aloe - A good idea just in case you get sunburned. If you forget to bring aloe or want something a little stronger than your bottle of Walgreen's brand aloe the massage ladies along the beach usually have fresh aloe leaves they will cut open and spread on your burn.
Running Shoes or Water Shoes - Gym shoes or water shoes are a good idea to bring. If you do an excursion into the mountains, to one of the waterfalls or just take a walk along the cliffs it's a good idea to bring a pair of gym shoes or water shoes. If you have an old pair of gym shoes bring them, if they get dirty or wet you can leave them behind. If they are name brand shoes it's not uncommon for someone to inquire about them; you could possibly use them to barter with. Something you may be interested in picking up both for your trip and for home is Vibram 5 Finger Shoes. A little on the expensive side but very cool....
Snorkel & Mask - Not a necessity but can be fun to use to hunt for seashells or look at fish.
Camera - You definately want to have some pics to remember your trip by.
Ipod - Good to bring on the airplane, while waiting at the airport and some tunes while sitting on the beach is always nice. If you have a docking station that takes batteries and won't take up too much space consider bringing it. Music for the room or to listen to on the beach.
Bandaids - Never know if you cut your foot on a seashell or rock, good idea to pack a few band-aids.
Passport - Obvious, can't leave the country without it.
Copies of Documents - While on the topic of Passports you should bring a copy of your passport to either have a travel partner carry or leave in a hotel safe just in case you lose yours. Not a bad idea to bring a copy of your birth certificate as well. If your credit card company provided you with an international customer service line bring these numbers as well just in case you have problems with your cards not working or lost cards. When packing all these things also take any unecessary items out of your wallet such as insurance cards, credit cards you don't need, etc. You won't need them and it's one more thing to lose or get wet.
Pen - You need a pen to fill out customs forms on the plane and they are hard to come by so pack a pen.
Bug Spray - For being a tropical climate surprisingly bugs aren't all that much of an issue in Jamaica but depending upon the season or after a long rain you may have a few mosquitos or sand flies and bug spray helps keep them away.
Benadryl - Good idea to bring for bug bites and other issues.
Map & Info - Bring a map of Negril like the one below. Better more detailed maps can be found on the internet by doing a simple google search. After reading guide books, travel blogs, and travel forums you'll have a number of places you want to check out. Bring info on restaurants and bars you want to see, a list of what bars have live music on what nights, and a map so if you overhear someone talking about the delicious snapper in coconut milk they had at Chill Awhile you'll be able to find it.
Backpack - Nice to bring on the plane but also nice to have for the beach and day trips to the waterfalls, mountains, etc. You may want to bring a change of clothes, bottle of water, camera, a snack, and a backpack is the perfect thing to carry these items in. Especially nice are the one strap "sling" style backpacks, very easy to get things in and out of without taking it off your back. Can't say enough about how convenient this style of backpack is when your constantly going into it for passport, digital camera, water bottle, etc. So much easier than a regular backpack where you feel like a turtle on your back when it comes to trying to get your stuff out. Columbia is a personal favorite for this style but check out the amazon widget to the left with teh scrolling products. There is a great slingshot style backpack in the scrolling products.
Advil - Good for those mornings after a few too many Red Stripes.
Tums - Always a good idea to have while traveling.
Money Conversion Chart - No one wants to do math on vacation, bring a conversion chart so you can quickly and easily figure out what your paying. You can get an chart up to date with the current exchange rate at www.oanda.com. It will look something like this below.
Put it into your wallet before you travel.
Epi-Pen - If you have food allergies, the closest hospital is Montego Bay which is about an hour and a half away and ambulance service isn't something you should count on.
Small Bottle of Iodine or Vinegar - Normally jellyfish stings aren't an issue but depending upon the time of year there's a chance of a sting and Iodine or Vinegar will take the sting away. Before you start worrying they are really small jellyfish and don't hurt much more than a mosquito bite.