Luggage and bags:
Don't make the same mistake that I did, and carry over a large back-pack, take the smallest bag that you feel you can carry. This will make things easier and quicker when you need to move around.
Clothing/Shoes/Weather Gear: Clothing to buy over here is very cheap, compared to North America, so keep in mind that you will be purchasing some things here. So, take as little clothing along with you, knowing you'll find some good bargains. And it will keep the weight down and your pack/s less bulky.
Photo Equipment: With the era of the digital camera, this has made things lighter & easier. We don't need to carry multible rolls of film anymore. If you plan on shooting a huge amount of photo's, consider purchasing an extra GB card for your digital.
But if you also like to still use your regular slr camera (film) be sure to take your extra rolls of film along in a clear see-through bag. A monopod, and or tripod is always a very good idea, this will ensure that your photo's are guaranteed to be sharp and in focus.
If you enjoy shooting with infrared film, you'll need to take your change bag, filters and extra rolls of film
Miscellaneous: It is a good idea to carry some small padlocks with you when you visit. It's just a little extra security, then your mind will be at ease.
Luggage and bags:
OK, so you're travelling around Southeast Asia, using Bangkok as your hub, and you want to know what to do with all of that extraneous stuff that you brought, as well as all of the gifties for the folks back home. What to do?
I left a bag of stuff, books and souvenirs and such, at Lek GH on KSR for a couple of weeks, at 10 baht per day. They put it into a locked room, with your passport number in the bag. You need your passport to retrieve it.
I also left a bag of things (now growing!) at New Joe GH on Trok Mayom, one block from KSR, for a couple of days, at 8 baht/day. They also lock the room, and secure retrieval with passport number. They seemed more systematic, with shelving and such it was quite tidy and organized-looking. I would definitely go there again.
Many guesthouses in the area store stuff, for about the same (or less, for longer term, like a month).