You need to make sure that you're getting (a lot) more calories than you burn.
Might be worth tracking you average calorie consumption/expenditure ... I have used
Fitday for this in the past.
The extra calories should be fairly clean ... easy to eat high-energy junk, but it's not going to be very helpful. Supplement with some good oils ... Flaxseed/Linseed is great (and excellent for you anyway), and Olive oil is very good for you. Milk is also an excellent way to get extra calories.