Carry the first-aid items that you are most likely to need, and are trained to use. I do a lot of volunteer work as an LCI and National Mountain Bike Patroller, so what I carry may be a little over the top for most, but over the years I've found there's a use for each item.

Here's a wonderful little resource to get you started thinking: http://www.utahmountainbiking.com/firstaid/ What I carry on a normal "just-riding-around" ride is pretty much the same as what they're calling the "biking boo-boo" kit, in a zip-loc baggie. Brave Soldier also markets a very nice, compact little kit that they call the "Crash Kit," focusing mostly on treating road rash & similar abrasions. Both fit very nicely in the center jersey pocket; you'd never really know they're there, but they are there when you need it. When teaching classes and leading group rides for beginners, I carry a little larger kit that has a few more bandaging items, some basic OTC meds -- such as aspirin -- and a CPR kit.

Probably the most important thing you can do is get some basic first aid training so that you can recognize problems, and take the appropriate steps to deal with them. The LAB "strongly recommends" that LCIs get some sort of first aid certification, and the NMBP requires it, so my teaching buddy and I both went to the Red Cross' Community First Aid course... probably one of the best uses of $65 I've done in a while. My annual recertifiation is coming up in May, and what the most cost-effective deal is looking like now is to go ahead and take the RC's instructor course... they're really looking for volunteers right now...

A cell phone is always a good thing to have (the response sequence is Check the situation, Call for help, then Care for the victim).