What I do is I load up my card with a ton of games, pick one that looks good and play it until I get to a part that has me stumped/frustrated. Then I just switch to a new game for a while. I repeat that for a few games then I eventually get around to playing the first and find that what I was stuck on wasn't really that bad. Then I play it until I get stuck/tired again.
That tends to keep me interested and keep the games fresh. Many of my games are mini-games or games that I can pick up and play during my lunch at work (Phoenix Wright, for example)
I mainly have a hard time dealing with the games that are trying to tell some epic tale (most RPGs), have too many little stats to manage (RPGs, monster collection games), games that have a horrible save system (Resident Evil DS, Mega Man ZX, etc) or games that require you to play everyday (Animal Crossing)