If you're like me, your NexStar stays on the tripod, standing ready for duty. Unfortunately, it collects a lot of dust while you are pursuing other interests - working, sleeping, eating, etc. For my NexStar 80, I found a great dust cover right in the closet - a standard pillowcase. I simply point the telescope straight up (diagonal between tripod legs) and slip the pillowcase over the optical tube and the hand control in its clip on the tripod. You might need a king size pillow case for some of the other NexStar models.
For the GPS models, try a bed sheet or a paper lawn bag available at hardware, gardening, and home improvement stores. The key is to use something that breaths so that moisture is not trapped next to the scope.