When I don't feel like sewing/quilting I don't. I don't want to force it b/c it's something for fun, not to be forced. So I will move onto something else I like to do. I'm an avid cross stitcher forever so I always have projects on the go (I pulled out one last night from 2004!!!) and when I don't feel like doing that either I usually knit a scarf. I'm also the type of person that has a book in each room so I can just pick up where I left off.
So I don't stress when I don't finish something, there is usually a reason, and I'm not ready yet to finish it.
Sometimes the best way to get back into it is to clean and organize, give the sewing machine a good clean out and oiling, tidy up the shelves, rearrange my quilting books and go through my fabrics and think about what looks good together. Usually by that point I have all my ducks in a row and can finish something.