the first time I went on lithium I gained about 18 kg (46lb). I went off and lost about 25 kg (64 lb). I stayed off it for a while but I was so suicidal that I decided to go back on it despite the weight gain (pretty crazy choice!!!) in the year since i have been back on it I have gained 10kg (25lb) and my weight is now fairly stable and only 3 kg greater than my starting weight.
the difference this time is due to monitoring thyroid function (Li can cause hypothyrodism - weight gain)
drinking only water - Li increases thirst - lots of calories if you drink soft drink/fruit juice
being careful about portion size - sometimes I used to continue feeling hungry even after eating.
I think if you are aware of the potential problem you are much less likely to gain weight, and it is easier to not gain it in the first place than to lose it!!!!!