plastic travel soap case (or any other small container that's durable and waterproof)
Okay, so start by grabbing your permanent marker and writing "First Aid" on the soap case. I suppose this step is optional, but it's helpful. That way, when you're in an emergency, you can say, "Hey! Grab the first aid kit out of my luggage!" And whoever you send to grab the kit will know exactly what he/she is looking at.
CDC and your doctor for the best ideas on how to stock your personal kit.