Practice going up and down scales. Why?

-Improve speed
-Improve accuracy
-Help you memorise scales for further solos

I always do about 10-15 minutes of up and down scales in every session. Also work on picking patterns or alternate picking whatever they call it :)

Try to jam on some midi tracks (Cheap and effective). Experiment during the summer.

You may also want to see other style of playing. If you're a metal guitarist, play acoustic or fingerpicking like I did. Try some Jazz or blues. Check out Terry Couch's section for some good licks. You'll never see country guitar the same way afterwards.

The main thing is to keep a discipline. You don't need to play 4 hours a day to keep your skills sharp.