Aside from the countermeasures already suggested by others:

Don't kid yourself into thinking that you're capable of mitigating or even self-diagnosing a hypomanic episode. This disease is so insidious because hypomania can be subtle and easily denied or explained away, especially by the hypomanic. It's the nature of the illness that you don't necessarily know when you're sick, let alone have the focus and presence of mind to take measures to 'prevent and stop' the sickness. Avoid stress, practice what I've seen someone on here, I think ~Christina, refer to as 'sleep hygeine', which presumably means consistent and sufficient sleep, and exercise/stretch/breathe to whatever extent you can, but never forget that you're never that far from the brink, and you can't count on your own brain to warn you when you're close. Rely on the constant analysis and assistance of professionals and the feedback of those around you whom you trust.