Have you tried the old breathing squats and pullover routine? I have heard it said that it can put twenty pounds on a broomstick.
Thorough warm-up. You want to break a mild sweat.
Pick a weight you could normally do ten reps with in the squat. Do this inside a power rack if you have one, otherwise find some reliable spotters. Then do a set of twenty reps, pausing between each rep to take three breaths, deep enough to move your shoulders. Upon completing the set, without any pause to rest, do a set of pullovers of at least twelve reps. This can be a fairly light weight.
Then gently walk around for at least five minutes to recover. The rest of your workout will be very basic -
[li] Bench press - three sets of eight[/li][li] Pulldowns or chins - three sets to failure[/li][li] Calf raises - three sets to failure[/li][li] Curls - three sets of eight[/li][li] Crunch situps - two sets to failure[/li][/ul] Perfect form on every rep of every set.
Do this routine three times a week. If you can manage it, do one set of each exercise in turn before returning to the first exercise for your second set, but otherwise don’t rest more than one minute between sets.
Don’t be afraid of aerobics, but if you are doing more than twenty or thirty minutes at a time, cut back to twenty minutes three times a week. If you can rotate so that you run one day, swim the next, and row or bicycle the next, so much the better, as it distributes the stress on your joints.
Your diet should be around 70% carbohydrate, 20% protein, and 10% fat. Don’t waste your money on protein powder. If you can’t stand the thought of not taking supplements, use non-fat dried milk powder - all the protein of the over-priced Weider stuff at a third of the price. If you are lactose intolerant, try non-fat yogurt. Five small meals a day is better than two or three large ones. Shun fat and sugar.
Two pounds a week is about as much as a normal person can gain or lose and maintain afterwards. Make that your goal.
Have patience. Pick up a couple of five pound beef roasts, and imagine that amount of new muscle added to your frame. If you can maintain your currently low levels of body fat, and add even a few pounds of muscle, you can transform your appearance.
Good luck. Ten pounds of muscle is better than twenty pounds of fat.