Breathe. Keep in your head, "don't rush". Write that on a drum head where you'll always see it. Think to yourself, "I'm too cool to get there too soon". Or "I'm too cool to get excited about that fill I've been working on". There's all kinds of mental things you can do until this isn't a problem. Count 1,2,3,4 in your head or under your breath. Our counting time is almost invariably better in time than our hands. I had (have?) a tendency to play in front of the pulse, but someone told me to try letting it drag me along. That was just a mental cue that kept me from being so in front of it, which can lead to speeding up. I think an awareness of how bad it sounds goes a long way too. Like, one of the things drummers tend to speed up on are parts where things build up (e.g. going into a chorus) and knowing how much it takes the wind out of the sails of the song keeps you from doing it. Speeding up on exciting parts can really kill the tension that you're building up to. I have to really watch myself sometimes.