What're some of the best ways of gaining weight?

I've been kinda underweight all my life it's not because of I don't eat I feel like I eat more than lot of people around me still I never really gain much. Ever since I turned 20 I started to loose weight which is kind of affecting some things. I once went to a nutritionist which didn't really work out very well so now any advices on how to not look like a complete stick