I've been struggling over this past year with trying to keep working on new projects while querying a finished one. The kind of constant level of rejection has been wearing on me. It's easier to handle the discrete moments of rejection with revenge queries and brief periods of wallowing in it. But in the long term, what really helps me is reading and re-reading books that inspire me. I recently read several of what were my favorite novels growing up, and I remembered why I wanted to be a writer in the first place.
That's my strategy for fiction and my academic work. It's grants season in my life right now and I'm getting a lot of rejection and dejection on that front, too. But then I re-read some of my favorite essays and books, and I remember why I love my work in the first place.
So, that's my strategy: finding ways to rediscover why I love this to begin with. Because, I think, in the long term, you have to love it more than you are crushed by rejection.