In my case, any drastic change in routine, weather, diet, activity level, sleep pattern.... tends to trigger a flare-up of symptoms. If you find this true for you as well, all lifestyle changes have to be made in "baby steps."
Dietwise, I've found that a moderately low-carb diet helps considerably. Sugar and starch are my kryptonite, so I have to be careful even with fruit and whole grains. Southbeach, the Zone, Atkins, Paleo, and other low to moderate carb diets seem to be helpful (in my own experience, and from what I've read). Though if you're change-sensitive you may need to make gradual changes, like giving up "white" or processed carbs, or even just cut back, rather than trying to overhaul your lifestyle in one fell swoop.
With exercise too, you may need to pace yourself carefully to avoid triggering flares. There are fibromites who run marathons, so you may be able to work yourself up to intense exercise, but you probably won't be able to start there. Some people find that exercise, even during a severe flare can be helpful in reducing pain. Others find the opposite, so you have to experiment, just start small.
If you can find a warm-water exercise program, I'd highly recommed it. The Arthritis Foundation or your local health departmenr, Department on Aging, or United Way should be able to help you find the nearest program. The closer the water temperature is to 90 degrees, the more comfortable it is when you have pain and mobility issues like fibro and arthritis.