Like much anything, consistancy is key.
As for nuitritional suggestions (I am hesitant to say 'diet', because this a long-term change in your eating habits, not just a diet fad), I have always been of the belief that moderation is key. I don't 'eliminate' anything per se from my eating, but I DO keep an eye out for caffeine and related substances. It STUNK when I kicked caffeine completely from my daily life, becuase I was taking in A LOT of it (3-4 cappacinos a day, 1 or two espressos, a ton of soda, etc). I was miserable for a week or so...but then realized that when it was gone...I was much more even. I didn't peak and valley anymore. I still take in some caffeine, but it is in moderation, and never at night.
So keep an eye on caffeine. Definately watch your sugar intake. I have been a big supporter of the merits of fresh veggies and fruit. I felt best when I ate healthy portions of both every day, with every meal. I just got a new wok, and I'll be usually it at least every other day to cook up yummy veggies. (you don't need to use much oil)
Exercising releases all sorts of good neurotrasmitters into your system that will improve your mood. I could write a book on it, but instead, just trust me.
Also, make sure to get consistant sleep. Having a sporatic sleep schedule screws with a 'normie' enough, but someone with bi-polar (or MDD, any anxiety disorder, psychosis related disorder, etc) it really does a number on the person.
I've gotta run, best of luck, and i'll check in later to see what other people say.
-pedagogue