For the past month or so, I've been working out more and at the same time I'm trying to gain weight (not fat, but muscle mass...rhyme not intended). I'm naturally a thin person, but I've noticed (for the longest time) that when I sit down, my stomach isn't flat. The bottom part (of fat) sags, I guess would be the right word to use. Are there any kind of excercises I can do to trim off the fat? And how many sets/reps should I do and how often?