There are so many cool things you can do with an old t-shirt. Here is one website that has a whole list of tutorials, from Bikinis, to dog t-shirts, tote bags, necklaces, bracelets, even yarn! There are probably more than a million ways to re-use an old t-shirt.
One of my favorite things to do is if I still want to wear it, is to cut it down and try to resize it. You can do this buy cutting little strips and then knotting them down the sides or down the back to adjust it to be smaller. You can also make a halter top out of it, there's probably a good tutorial on that website.
Threadbanger.com has a lot of tutorials for stuff like this and they have videos that show you step by step.
What I like to do when I find my t shirt or any shirt in general has gotten to big for me is start at the bottom and measure about maybe half the way up and then cut up at an angle to give it a kind of funky edge. You can then wear it over a tank top or another t shirt that's better fitting and it makes a cute combo. You could also add a cut shoulder to the mix if you like..or just do either one by itself.