I feel for you, BLM. I am in the same boat you're in. I have no diagnosis yet so my doctor hasn't put me on any meds yet (I'm going to see my GI next month and am going to ask if he can try me on some meds). I have been ill for close to a year now, and I've lost about 15 lbs in that time (I'm a fairly tall female, who was about 135 to begin with - so I started out skinny, and now I'm at about 120 and really skinny). I try to eat protien every day - either poultry or fish - and I drink an Ensure every morning for the extra calories and nutrients. I try to eat as much as I can when I'm feeling okay, because I know I'll have no appetite when I'm feeling bad. But still, I can't seem to gain any of the lost weight back. And I'm still slowly losing weight, maybe about .5 to 1 lb every month.
So long story short, I also don't know what else can be done to regain weight, other than getting on meds such as pred that promote appetite, like Kelly said. (The really sad part is, often times people will see me and ask how I stay so skinny - if only they knew!)