1. insect repellent
2. flashlight (for the indigenous parts of the country and walking around at night, which you shouldn't do alone anyway but i did)
3. batteries (for your flashlight, CD player, etc)
4. portable or handheld fan (i didn't realize i was going to the mountains so this was just extra luggage for me, but i'm guessing it's hot over in el salvador)
5. as few clothes as possible, but as much as you find really necessary (i was surprised i managed to fit EVERYTHING in a carry-on)
6. digital camera, extra memory cards (i always had it around my neck in a camera bag, took the most amazing shots .. i was the only one who brought a digital camera, and i can assure you i got the best shots .. film is such a hassle and can be damaged)
7. $$$ (i don't know about the currency in el salvador, you should look that up .. i only needed $250/mo in GUA for extraneous things like gifts .. food/lodging/travel were taken care of .. bring extra for a bus/cab if that's not covered in your trip)
8. a journal
9. any medicine you need
10. other things you normally need when you pack for any trip