Some of the best exercises to gain weight are your main lifts. Squats, bench press, shoulder press, dead lift etc. but how are you performing those lifts? A lot of guys, especially in football do those lifts and try to just get the weight up. That’s going to get you stronger but it won’t put on size. You have to do a weight that you can handle with perfect form to put on good weight. Also you shouldn’t be doing those lifts at a low weight. If you focus on doing 10-20 reps at a time you will get lean no doubt, but you won’t put on size. That’s a very high intensity training method that football players who want to gain weight shouldn’t do. Heavy weight that you can handle, with perfect form, in the 3-5 rep range. That’s the way to put on size in the weight room. And honestly those main lifts we mentioned above are the best ones, but you should be lifting like that for all exercises. If your goal is to solely put on size, you need to lift properly with all exercises. Now I’m not saying you can’t throw in a burn out set every once and a while but especially with those main lifts it should be heavy and a weight you can handle.