You shouldn't assume that you'll get mastitis. The chances are that you won't. Try to stop gradually, rather than from lots of feeds each day to nothing suddenly. That will help. Your supply will naturally reduce in line with your reduced feeding.
Expressing a little to relieve extreme pain and engorgement is a good idea. It will help, but leave it several days to a week between expressions to avoid lengthening the time it takes to become dry.
It depends how much your DD feeds now, but the chances are you'll have no issues at all and no pain. Good luck!
For what its worth, DS is nearly three, and we've been down to one feed a day for a while now. He didn't feed for three days in a row recently, and I wasn't engorged at all. I know I still have milk there because I hand expressed a little to wipe on a graze he had.
So I'd say, as long as you're not stopping from like, six feeds a day or something, then you should be okay. Your milk will dry up gradually, but don't expect this to happen as soon as you've stopped; it can take a little while.