At the Dome, a lot of folks will tie the things they want autographed (caps, balls, helmets) to a string and dangle it down from the stands as the players walk from the field and through the tunnel toward the locker room.
From what I've seen though, it's a gamble to get many autographs if you're seated up there. I don't know the specifics, but I'd recommend checking into the thing they do that allows you to get sideline passes. They run some program in the parking lot, I believe through the True Blue fan club, where you can pay to play a few contests (like those things where you spin a dial and you win on whatever it lands, I think). You can win sideline passes that way. And a lot of times, if you play enough times (ie. pay enough money) without winning, they'll just give you a pass out of pity lol.
Oh, and if you are sitting in the grand stands, get there way early.