how do i increase my calorie intake without gaining weight?

ok... so, for the past 3 to 4 months i've been eating about 1,120 calories a day (yes, i know that's not enough!). my question is, how do i increase my calorie intake without gaining the weight i've already lost back? should i start by adding 50-100 calories a week? or something like that?

by the way i'm moderately active. i dance 3 times a week and take walks quite a bit. i'm 5'4" and 117 pounds on a small frame. 

i know what i've been doing hasn't been that healthy (i should be eating AT LEAST 1,200 calories a day), but i would like to start raising my calories, thus my question.

omgsh this really helped me a lot because ive been eating a dangerously low amount of calories for about 2 1/2 weeks

i was really emotional and afraid to increase my intake in fear of gaining weight but now i know that its really not that bad and its much much healthier