This may not be what you should be thinking during the point, but remember that in long rallies your opponent will also begin to feel the same way you do.
Many people (including myself) feel like they must do something with their next shot. This impatience often causes people to over hit or make a mechanical error. In long multi-stroke points, you have to learn to be content with just hitting rally balls and reacting to each shot. Don't worry too much about the pace of your shots, but pay attention to the depth. If you have to take a bit off your shots and make it a bit loopier, that is perfectly okay. The only time you should change your mentality is when the opponent hits a crappy shot that is easy to take advantage of.
If you find your shots are waning a bit in power, try not to think about mechanics. Think about how you want more depth and how you have to appropriately move to be in a good position for the next shot. The only mechanics you should ever actively think about with rally shots are court positioning and maybe rotation. If you begin questioning the way you swing consciously, it will lead to errors.