As some of you know, playing the ukulele is both a hobby and passion of mine. As with all stringed instruments, putting on new strings is one of the cheapest/easiest things you can do to improve the sound of your instrument. My new uke, an Oscar Schmidt OU-5, had mediocre strings on it. I changed them for Martin strings recently. Here are some pieces of advice that might help you next time you go to change your strings...
-Watch this video on youtube a bunch of times: http://www.youtube.com/watch?v=mU42wHlS-NY
-Remember to only change one string at a time. The other three strings will keep the tension on the neck and prevent damage to your instrument. They also allow you to have a reference point when making your knot.
-The knot will tighten as you wind the string on—be sure to leave slack at the bottom. The slack will also help you hold the string better as your fingers will probably get sweaty/slippery.
-Leave plenty of slack at the top, too. Chances are that the strings are much longer than you need them. You can always trim them after.
-When winding the string on, make sure one round goes above the hole, and the rest go below the hole.
-Use a string winder—no need to get carpal tunnel...
Is that helpful? Good luck!