I've been to Jamaica a few times. It's gorgeous, frankly, with the world's most magnificent coffee and rum; it is a striking place, with the resort life one way, and everything outside the resorts 180 degrees different.
It's been a bit tricky to pack for this kind of vacation. Each time I go, I learn from my mistakes, so here's a compilation thus far:
Clothes. Jamaica is hot and humid. Not quite as humid as the American South, perhaps more similar to the general East Coast/Midwest type of humidity. But it is as hot as the South.
Short-sleeved tee shirts don't work. I brought a bunch the first time I went, and they sat in my suitcase. Cotton tank tops are good, or sleeveless, thin cotton or rayon tops (I have a couple East Indian ones I bought there). If you've got to do short sleeves, make your top loose-fitting and as lightweight as possible.
Generally, you wear a bathing suit all day. The styles tend to be pretty modest, no thongs or teeny strings, just regular one- or two-piece bathing suits. Along with this, you'll need a skirt or sarong or something similar to stash in your beach bag...some restaurants don't consider you "dressed" in your bathing suit alone, you have to put a skirt over it. (Men are considered "dressed" with a shirt over their swim suit.)
You can carry one bathing suit and just wash it out at night. No one seems to notice if you wear the same suit every day. Swim suits take up little packing space however, so throw in another if you like.
You do need to bring a couple of long-sleeved shirts just in case. It got "chilly" last time I went, and I had to wear the same solitary long-sleeved shirt I'd brought.
No one irons there; some wrinkles in your clothing are okay. I brought one of those travel clothing steamers...it sat in the suitcase along with the tee shirts!
Evening wear...you dress up every night. Again, it's hot, so "dress up" does not mean the same thing as it does here. Something like a pretty, sleeveless, lightweight dress, or similar top and skirt, would do it. You can switch out your tops and skirts somewhat, but try to think in terms of clothes that don't wrinkle, so's you can squash more of them into your suitcase.
Shoes. It's too hot generally for stockings and pumps, although you can wear them in the evening if you want to. Sandals are accepted footwear day or night; make yours lightweight sandals. Bring some water socks (in a pinch, you could buy them at the resort).
You will need a comfortable pair of "walking sandals" for jaunts outside the resort; wear these on the plane coming and going.
Hair. You will need to put up your hair unless it's short enough not to hang on your neck. Bring enough hair doodads to keep you happy putting up your hair each day.
You will need to bring a mild shampoo as you will wash your hair once or twice per day. I washed mine in the morning and evening (you could skip the morning wash, but you will need to wash your hair after going in the sea or pool). The complimentary resort shampoo ain't going to cut it.
Likewise, bring a sufficiently rich daily conditioner.
Umbrellas. Some resorts have umbrellas, some don't. If yours doesn't, bring a travel-sized umbrella.
Makeup. During the day, forget about it. Eyeshadow instantly melts; blush is unnecessary; no one seems to wear makeup anyway. If you like, a little waterproof eyepencil or mascara and lip balm (tinted or un-).
In the evening, you can wear eyeshadow, blush and lipstick as you please, but you won't feel like heavy or dramatic makeup.
Perfume. This is like the South: light scents disappear within half an hour. That's why Southerners bring on the heavy, sweet scents, stuff that might well knock you on your head in drier, cooler climates.
Jewelry. People don't wear the flashier, more conspicuously expensive pieces. You'll see a little gold, maybe small gemstones, but just as much "bridge" jewelry. Silver and semi-precious are fine.
Some things I'm still working on...no see 'ums. These are tiny, biting flies or "midges." They're worse than mosquitos in the sense that the bites don't start itching insanely until a few days after you've been bitten. And the no see 'ums are tiny, unlike your big, slappable mosquitos.
From what I've read, the no see 'ums live in the sand on the beach. When the sand cools off later in the day, the no seem 'ums come out and bite you. It seems as if DEET is the choice repellent for these, but the DEET spray I'd brought last time didn't seem to discourage them much. Thinking of a higher-concentrate DEET lotion, unless there's something better around.
Of course there are the obvious things to carry: sunscreen, hat, sunglasses, et cetera. The sun isn't as strong as it is on a hot day in California, but, as soft as it feels, you'll still burn without sunscreen.
There isn't much shopping in Jamaica. They have little malls with some jewelry, clothing, of course the coffee and rum...well worth visiting, but you won't bring back bags and bags of stuff.
On the other hand, prepare to lose some weight and get more buff, as most of what you'll "do" over there is physical: swimming, "kayaking," walking on the beach, pedaling an enormous sea tricycle, scuba diving, and so forth.