Be sure to maintain your first aid kit:

First, go through it after each trip and replace what you used. (Of course, if you didn't use anything, no need for this.)

Second, go through it once a year and replace the medications. It's best if the labels for the medications (I use computer-printed slips that I put inside tiny ziplock bags with the pills) have the expiration date on them. I do this each winter and replace anything that will expire in the next year.

Third, examine the bandages, especially those in sterile paper wrappers, which with time and squashing into the pack tend to get ratty and therefore unsterile. If the wrappers are turning yellow with age, it's definitely time to replace the bandages!

Fourth--maybe this should be first!--reread your first aid text (assuming it's of recent vintage) and try to update your info every year. I'm lucky in having a son-in-law who is both a backpacker and an ER physician. You can get a lot of info online, but as I mentioned earlier, be absolutely sure it's of very recent date, not undated and from a reputable source. Of course be sure that you take a CPR refresher course as indicated on your CPR card. Frankly, you're far more likely to need CPR skills back near home (such as your workplace) than out in the wilderness. If someone has a heart attack out in the wilds, there's not much you can do if help can't get there within 20-30 minutes. Without a portable defibrillator (far too heavy to carry backpacking), I've been told that the chances of saving a heart attack victim with only CPR and no prompt hospital followup are less than 5%. For lightning strike and drowning, though, CPR can really make a life-and-death difference!

Finally, do not try to perform first aid on anyone else until you get training. A recent court case (here in Oregon) considered this negligence and that Good Samaritan law didn't apply. The case was where some completely untrained bystanders moved someone with a spinal injury without first stabilizing the spinal column, with the inevitable results.