Plastic key caps are a great way to ensure you can easily tell your keys apart. If you'd like to make your own, Instructables user Andrea Biffi shows how to do it with some Sugru.
All you need to do is cover a key with Sugru and flatten it down. Then, you'll want to add some texture to differentiate the keys. Biffi recommends using something like textured cloth to imprint on the surface. It's a simple process that will easily make your keys unique and easy to find.
If that's too much work, an old balloon will also do the trick.
Sugru Key Head [Instructables]