There are so many really cool cell phone cases out there...many of which are also really expensive. I have found that creating my own can be considerably cheaper.
I look for discounted cases in clearance bins (for really cheap!), and if I do not like the color, I repaint it. ( I always have spray paint left over from other projects.) I print my illustrations on label sheets, stick into place, and spray a clear coat of gloss as a sealer on the whole case. (Mod Podge works as well.) I like to use label sheets because I can also print photos on them to use.
Doing this to an old case you already have that is getting worn, or a case you are just sick of, is even better!
Eye Phone!
It is also fun to glue items to cases, even if they do not always want to stay on.