Track Yourself

I've been saving my race numbers from the past 11 years or so, and put them up in my garage.
You can keep track of your events even easier than that. If you're a runner, walker, trathlete or cyclist, you can get a list of the events that you've done from Athlinks.

It doesn't find all of the 5K's and it misses a few older events, but it's the most complete list that I've found. Here's a link to my event list. You can go there and sign up for a free account get a list of your events, There's also a feature that let's you claim "lost races" that Athlinks didn't automatically put on your list.