My best results took me from weighting 135ish up to 153 over the course of about 5 months by adding a quart of chocolate milk as my post training meal and following stronglifts 5x5.
Once I got close to my strength goal I switched to a more Paleo diet, but kept lifting. My weight went back down around 140 but my strength stayed.
Here's another vote for squats, deadlifts, and milk. Personally I prefer Starting Strength (particularly the book, not the wiki) over StrongLifts, but the core is the same. Lifting heavy and eating big makes bigger and stronger.
This is tangential to the topic of the thread, but why would anyone want to gain weight longterm unless they were medically underweight or had an eating disorder? I've even read that many people at a "healthy weight" are actually still too fat.
Eat MORE, sleep MORE, lift MORE
:) alls been said but my emphasis on more.
KEEP doing it
Yeah, Until I was twenty fucking FIVE I could wear kids sizes clothes if I wanted to. That's SCRAWNY.
Now at least I wear adult clothes, but still, at 5'7" and 140 I have a 28 inch waist.
Edit: Skinny guys really need to get over visible abs thing if they want to get bigger. You almost have to add fat along with the muscle and then sometime later (MUCH later-years if you do it right) diet back down. The trick it to not add too much fat compared to muscle. It's called "bulking."