I've gotten the winter time blues before, but never really in the summer.
I don't know if any of these things help because I have never had a need for a cure, but some things that I can think of are taking a trip (camping or something), playing guitar outside, making sure you have a little free time to hang out with friends or do something that you like to do, remembering all the money that you are making if you are working a lot, and finding time to relax.