That is always the real bummer for me when the weather gets hotter. I hate putting away the sticks, so I usually think of other projects I can knit during the summer months. I work on Christmas presents, dishcloths, table runners, lacy socks, cotton knit camisoles, etc. Right now I'm working on a light cardigan that I can throw on when the nights get a bit cooler, which they occasionally do around here until mid June. Nothing heavy. Just something to cover my arms.
Another thing I'm going to try my hand at this summer is hand dyeing and spinning of wool. I hope to buy up some plain roving that I can dye and then practice some spinning on a drop spindle. Hopefully I'll get the hang of it in time to make some lovely slub yarn for my fall and winter knitting projects.