Best ways to gain weight?

Eat more calories.: Eat more calories than you burn at rest and through exercise. Do not give up your exercise, but concentrate on resitance exercise ( weight or weight machines) so you can build muscle. Avoid filling foods with few calories, ie popcorn, lettuce, etc. Eat more meat, cheese, and protein sources with fat. I do not recommend eating sugar/fat combo foods like ice cream or brownies.

Eat, eat, eat.: You can gain calories by just eating more food. People tend to gain weight with meat because it's more calorie-dense than other foods, but you'd gain more weight by just eating more of less calorie-dense foods. Just keep it healthy - there's a difference between gaining muscle weight and just getting fat. And exercise as well, so you gain lean mass and not just get fat.