I use the itty-bitty ziplock bags you find at craft stores (usually in or near the bead section). I put a label in the bag with the pills which includes the name, amount of active ingredient, the dosage and the expiration date. For example, "Acetaminophen, 500 mg., 2 every 4-6 hr., limit 8/24 hrs., exp. 10/10." These labels are on my computer, so I only have to change the expiration date and don't have to look up how to spell Acetaminophen every time. :-)
The itty bitty bags are held together in a ziplock snack bag in the middle of my first aid/essentials bag, so they're protected from light and from crushing.
I try to replace pills, bandaids and other items that have been used after every trip. Once a year I do a thorough inventory of the first aid/essentials bag to replace anything that has expired or is really close to its expiration date.