I've gained about 15-20 pounds since I started working full-time and got married, but I was a little on the skinny side before, so now I'm about normal. Lifting weights, regular exercise and a good diet helps, though
I'm not totally in agreement with the whole Atkins thing, but there is one very strong point to it that he took to an extreme. If you're sitting around a lot, watch your carbohydrate intake. You don't need much. Try to eat less bread and other such starchy things, and more than anything, cut out sugar. Just cutting out crap like high fructose corn syrup (i.e. soda) can really help.
More vegetables, more fiber, you get it. Try to exercise when you can. I don't have a lot of difficulty making myself exercise anymore, because it is my antidepressant. I am one that tends to feel depressed for no reason I can understand, simply due to an imbalance of chemicals, and exercise naturally balances those chemicals out. I didn't believe it when I was told so, but it's seriously amazing how much of a difference it makes. And running for 30-60 minutes late at night when there's nobody around and it's totally quiet = awesome.