Yesterday, a painting friend of mine who had just moved into oils was lamenting the time wasted between finishing a painting and starting a new one. I had to sympathized because I, and probably MOST painters, have encountered the same lull. Sometimes it's only a day or two, sometimes it's been known to stretch into weeks or months. Not only has this happened to ME, but I use to see it OFTEN when teaching high school kids to paint. They would even drag out the completion of a painting interminably, basking in its success, just so they wouldn't have to face the uncertainties of a NEW project. That's really the psychological problem involved.
Personally, something that I've found works is to start the preparations for my NEXT painting before I finish my current one. I usually have the photography and computer work done, sometimes the painting surface prepared, or even the preliminary drawing on that surface done for my next painting before I finish the previous one. This has the added advantage of breaking up my routine, allowing me to work on different stages as I feel like it. If I don't feel like painting, I do some other task at some other stage on some other work. I've even been known to get to "into" the NEXT painting so deeply that I'll start it (maybe even FINISH it) before finishing the one I was into first. I never USE to do that, but since I've retired I sometimes do. At any rate, I find if I've got two or more paintings "in the works" at the same time, it allows me the freedom to pick and choose to do whatever "turns me on" at any given time.
I've also found that making a list of paintings I want to do helps too. You don't have to necessarily go down the list and do them in order, but it helps to keep your mind focused on long-term goals. Sometimes I'll just make a stack of photos or sketches I've done and pick and choose from that. Of course this assumes you have more "ideas" than you'll ever POSSIBLY get done. Sometimes, THAT'S the key. Its all to easy to get so involved in doing the art WORK you tend to slight the art THOUGHT until you "hit the wall" so to speak and find yourself both "workless" and "thoughtless", in which case, the doldrums set in and all kinds of "blocks" come into play.