I find a B complex to be helpful sometimes, e.g., when I'm depressed for an extended period of time--however, because I often rapidly cycle, it can also backfire if I haven't been taking it regularly (hypomanic + jolt of B vitamin cocktail = off the walls).

Other than that, doing my best to keep a regular cycle including diet (especially excluding almost all artificial sugars), exercise, good hydration (always amazed how important that is), and moderating my intake of caffeine and alcohol.

I also find it helpful to be conscientious of my moods... if I feel myself start to get low, I'll change rooms or get out of the house for a while. If I start to get amped up, I'll straighten up and clean the kitchen and then force myself to do something active, but focused & goal oriented: like playing darts or some random game I've invented around the house. It doesn't always help, but it can sometimes reign me in, especially if my moods seem related to some sort of social anxiety.