I've found that most chess games that I play are won are lost, not because of positional imbalances that gradually favor one player over another, but rather are the result of someone not seeing something on the board. Maybe the mistake sets off a chain reaction that leads to a bad trade (i.e. rook for knight) several moves down the line (yet still forced), or worse still, a move deserving of a ?? on the move list.
The weird thing, though, is that these games happen in untimed settings. I don't feel the need for a timed game when I'm casually playing at home, and many times, if an interesting position shows itself, we'll set up a branch game and see what happens with different moves. This is all to increase learning. So, why do we make mistakes? My theory is that mistakes result from inability to divorce oneself from his or her pieces, initial development, and the pressure, or sense of urgency, that your opponent is placing on you.
They're My Pieces!
In my chess career, I have to admit, I am inclined to think that I have the upper hand and the momentum, and that my opponents moves are defensive responses to my own moves. While this is obviously sometimes true, I've found that if you go into a game thinking that you are better than your opponent and see more, you usually don't. There's no magic to this situation, it's simple human nature to think that we're better than others, but in chess, when you have soldiers on the line, you have to think of them first.
I've found several drills can help with this. The first is to play with your opponent, rather than against him or her. This doesn't work in tournaments, obviously, but in a casual setting, play a move, discuss what you were thinking, and then your opponent asks what you think his or her best move is. You isolate your weaknesses, find where you're strongest, and gain a grasp of another play style all at the same time. This can be ported to an actual competitive game, though. Instead of talking through the game with your opponent, just talk it through with yourself; imagine that you have a second voice in your head telling you where you are weak and where you are strong, and realize that this is shaping how your opponent moves. If you only consider your opponent's moves based on your strengths, you will be in for a nasty surprise.
I got there first
I'll be the first to admit that I have a very aggressive style of play. I try to hold onto my pieces visciously, and I rarely take first blood. That being said, I develop and subsequently advance very quickly. What does this have to do with mistakes, though? Players with a lackadasical play style, who are more interested in defense than they are in attack tend to be forced into situations where they have limited moves, much less good moves. The key, in my (very short, not all that knowledgable) book, is to develop while defending. See the following position.
Now, this puts pressure on black's center pawns and develops pieces. I'm sure this isn't the best example, but I'm on lunch and typing quickly, cut me some slack
Everyone cracks under pressure. I may be a good chess player, but put me in a game against Kasprov, and I assure you that my play will suffer because I know that my opponnent has the upper hand, right from the start. The same is true when the game is drawing to a close, if you are down in players. I think, though, that there's a reasonably simple way to stop making mistakes in this case. First of all, your opponent is probably feeling pretty good about him or herself, and therefore may start to get sloppy (don't expect the pros to do this, though), and secondly, you have less pieces with which you can mess up.
When I look at a big board filled with pieces, the complexity is absolutely mindboggling. If, however, I have been reduced to less than 10 or so pieces, the action becomes more managable. So what, your opponent may have better pieces than you, or his or her position may be superior. This is the time for brilliant comebacks like you see in cheesy kid's movies. Begin to think of your pieces as a small, close knit task force rather than an entire army, and focus on how they communicate with each other, rather than how they communicate with you, and you'll see possibilites open up (it sounds weird, but try it before you knock it).
When the pressure's on, your opponent only needs you to make one faux pas before you're hosed. Don't make that mistake.