Mr. Wiggles: I'm the same way with Peach in MarioKart. I've had the game since 1997 and my friends used to always poke fun of me for using her. But I've found I'm actually good with Peach--she's fast and handles well.
So I'll go through the games I have and see what secrets I can come up with:
Super Mario 64: On the water levels I've found you can get cheap life if you go under the water and come right back up even if your damage wasn't from being underwater. It's the only way I beat that one with all the faucet taps in it.
Mario Kart 64: Not only do I use Peach, I also am big on laying bananas in certain places. I've found that there's a hill about 3/4 of the way through the stadium where you can get guys to fall if they slip on one, there's a place like that on the Bowser castle as well when you come up the bridge. I also have found that mushrooms can be used to blast through a tunnel on the beach and to get up that rugged off-road terrain of the Peach castle circuit that most people seem to fall into except the computer players. There's also the jump on the stadium that has probably already been discussed here, most people I play against seem to know what it is but not everyone uses it (I try it a lot, I have about a 50% success rate). I consider MarioKart 64 to be my best game.
Mario Kart Double Dash: My secret here is to play at 50cc if I want any chance of winning. I'm that bad at this game. It's fun but I'm just not very good!
Madden: To win here you really need to have a good knowledge of football. I might not be the best at playing the game but I know my formations pretty well, especially on offense. If I could just execute them a bit better on the controller I'd be a champ. Don't always go with what Madden recommends, either. Rule #1 is that you need a good mix of run and pass. And don't be afraid to try gadget plays or different formations (like one with two halfbacks). My biggest strategy, though, is using the QB sneak for a cheap one yard run. I've gotten out of some sticky situations with this. The defense doesn't seem to anticipate it unless you're at the goal line but even there I get it in 8 of 10 times.
Baseball (any baseball game): I've played more baseball games than anything and have more tips on this than anything else. I'll use MVP Baseball 2005 here since I play it the most now. First, you need to pick a solid team that has a good mix of hitting, pitching, and defense. If you lag in one area make sure you can support yourself somewhere else. My teams tend to have weak benches as a result of me spending all my money on starting pitching and solid hitters. You will also want to find a solid closer--preferably a guy who throws a good fastball. When playing, keep an eye on the pitcher's number of pitches. Anything above 110 warrants removal if he looks tired (you can tell when his control gets off). Some guys can last longer than others, check the stamina attribute. Make sure you have both a lefty and a righty in the pen at the same time if you aren't sure when you need to bring a guy in since a lefty batter is more likely to fail against a lefty pitcher.
Your lineup should be arranged something like this:
1st: Guy who gets on a lot, walks a lot, can steal some bases
2nd: Needs to be able to bunt, speed is nice, usually a decent contact guy
3rd: Your best all around player, hits for average with occasional power
4th: Put your biggest HR threat here
5th: This usually goes to your other big HR threat
6th: This guy, to me, is the "lite" version of the #3 hitter--all around type of guy
7th: Put your weakest hitter in this spot unless you're playing with an AL team
8th: On NL teams, put a good contact guy here
9th: With an NL team, this is your pitcher, with an AL team, your weakest hitter
Also, alternate your lefty and righty batters in the middle of the lineup. It makes it tougher on the other team's bullpen late in the game.
Pitching-wise, you want your best starter as the "ace", a solid second, third, and fourth starter, and a fifth starter who you aren't afraid to skip now and then in the rotation. Try to go righty-lefty-righty-lefty-righty (or the other way around) if possible. Even if you are unbalanced (like only one lefty, for example) make sure he's somewhere to break up the righties (if I have one lefty I usually put him second).
In your bullpen, you will have to decide if you want six or seven guys. Go with a seventh if you have young and inconsistent starting pitchers. One guy is your closer (usually your best fastballer, he comes in during the ninth), two are the setup men (a righty and a lefty who come in during the eighth), you need two or three middle relievers (they pitch in the sixth or seventh if a starter can't go that long), and you need a "mop up man" who comes in if your starter gets killed (i.e. gives up three home runs in the third). He needs to last for a few innings and also can sub for a starter who needs to miss a day due to injury.
Finally, your bench. You want at least one solid lefty batter and one solid righty. You will have either five or six guys depending on how many are in your bullpen. One of those is a backup catcher, you usually want two infielders, two outfielders, and if you have six guys, either a third outfielder or a guy who plays both outfield and 1B.
If you want any other baseball tips let me know!
Tetris: This is on my computer, but I usually leave a large opening and wait for that long piece to come down.
