Make it into a game
I'm an alumnus of Mary Robinette Kowal's Writing on the Fast Track course. Last November she set up a Facebook group for all of her students and this year I devised a game to keep us all putting ourselves out there: at the end of the year I will send out two big slabs of chocolate for the person who has managed to gather the most rejections this year.
Author Kevin J. Anderson has an actual trophy, the Writer with no Future, because he had the most rejections - by weight - out of all the authors at a particular convention (750- to over 800 varying by the telling). My personal goal in life is to take that trophy away from him. So, Mr Anderson, be warned!
So shake your, hopefully metaphorical, tail at the rejectioneers and rejoice in every one of them. Then submit your work onward.
Friends can help
If you've had a particularly bad disappointment, even your non-writing friends will most likely be there to buck up your spirits with pictures of owls, kittens and fennec foxes. As well as some sage advice.
Writer friends are especially helpful at this point since they will have been through this too and will readily offer you whiskey and chocolate to help you recover.
If the chocolate and whiskey don't do the trick you might get the overwhelming need to punch something.
In any case, physical exertion is very, very good for you. Working out causes your body to release a chemical called endorphin, which is apparently related to morphine. And much like morphine, it will create a positive feeling, often described as "euphoric" in your body. Just without the negative side effects of, you know, addiction and eventual ruination. So embrace your violent side and sign up for boxing lessons.
Have a good cry and move on
Sometimes you just have to mourn the loss of something that was never yours to begin with. Put on a sad movie if you like.
Then the next day get your story out for the next round of rejections.
It'll be easier next time
Maybe not. Okay, so maybe this isn't a perfect plan. But I'm working on it.