I've tend to have a few days each year that I really hit gold with yard sales. Mostly, though, the best I do is $1 wii games or something equally disappointing (but at least I don't come home empty-handed).
Edit: I do live in a fair-sized suburb next to the capital city, so there is definitely a lot more action here than in more rural areas. But there is also more competition, so it balances out.
Here are a few tips that work well for me:
1) Go on FRIDAY. Sales in my area are usually Friday and Saturday (when I was younger they used to be Saturday and Sunday). Obviously being the first to get there will increase your odds of getting the good stuff. This is the first year in a long time my work schedule hasn't allowed me to take Fridays off, and it has been a lot tougher with people telling me "I got rid of all my game stuff yesterday".
2) If you don't see anything, ASK. Almost ALL of my big scores have come by asking people (who didn't have anything set outside) if they have any old game stuff. You'd be surprised how often people say "I have a box of old games but didn't think anyone would be interested" or even "I didn't even think about that, thanks for reminding me."
3) Don't pre-judge. I've been tempted to drive past certain "types" of people (and entire neighborhoods) based on age, appearance, or some of the other junk they're selling, but games and stuff can be found in the unlikeliest of places.
4) Set limits. Not just spending limits, but time/distance limits. Tell yourself you'll go our for two hours and head home no matter how good (or bad) your day goes.
5) Take a friend. It's always more fun with a friend. Plus you can play off each other, have fun, and make better deals as a pair. Most of my biggest days have been with a friend. Happens to be the same guy every time; I'm starting to think he's good luck!
6) Have a consolation plan. Don't get discouraged. You'll strike out maybe 90% of the time, and the odds of actually scoring high-value games (as opposed to low-dollar titles and commons) are a lot less than that. Don't expect much and you won't be disappointed. I try to stop by one of the retro shops in the area if I strike out after a day of yard sales and treat myself to something with the money I was planning to spend. Don't get discouraged; it's a numbers game and is all luck.
Also: pawn shops, flea markets, etc might have some good deals, but they are usually picked over by collectors frequently and the sellers have a better idea of what the games are worth. Goodwill-type stores (at least in my area) tend to sell the games (especially cart games) online and the stuff they do have on the shelf is usually overpriced. But I've still had a few lucky finds.
