How to avoid blunders
Tamsin: Since I'm one of the world's leading experts of making blunders, I thought I'd share some of the ways I've tried to make myself less of a trailblazer in this activity. In other words, here are my anti-blunder tips:
1) Sit On Your Hands. Uncomfortable but it restrains impulsive moving.
2) Think Before You Play. Pretty obvious, huh? So obvious that many people play quickly without thinking, relying on instinct and emotion. I shudder when I recall all the times when I launched an attack impulsively, only to find it made my position worse, or when I played out a losing capturing race from inertia, without considering its value as aji. Go is a game of thought, and making the effort to think about your moves can only help you to play better. But it is an effort, which is why so many people don't do it.
3) Use a Squeeze Ball or Similar Object. Like sitting on one's hands, this prevents an itchy mouse-finger from clicking an unthinking move. It also relieves stress.
4) Use Checklists. At its simplest level, you could make a two-step list to apply on every move, such as "1. Where would I like to play? 2. Am I missing something?".
5) Ignore Your Assumptions. When playing situations arise and are resolved to a certain extent before play moves to another area of the board. When play shifts, I end up with assumptions regarding the status of the stones in that area of the board. After a while, all areas of the board have a status assigned to them. Before moving it is good to set these assumptions aside and view the position afresh. This prevents silly things like self-atari among other blunders.
6) Put Your Blunders Behind You. It really hurts to lose a well-played game because of a blunder. But it happens to everybody sometimes. You've just got to accept it and move on to the next game. Allowing your memory to burn with anger only clouds your judgement and makes future blunders more likely. Be like O Meien. When he missed the famous ladder in the first game of the Honinbo match of 2000 he burst out laughing, even though he confessed later that he wanted to weep. Again, he suffered a spectacular mishap in the first game of the 2002 Oza title match. Once more, he laughed at his misfortune, although one can believe he was torn up inside, as he would have won the game by 0.5 had he reinforced his position. But who went on to win these matches? It was O Meien in both cases. So, when it all goes pear-shaped, try to laugh at it and do your best next time!
7) Don't Move (and Think) Until You're Uncertain. Quick play occurs often because we feel certain about our move so we play it immediately. Then we see how foolish it was. Doh! A method I use now is to think until I am uncertain. Sounds funny but here is how it works. I see where to make my next move and after some amount of time (5 seconds, 30 seconds, whatever) I am confident in that move. It is the move I will make, I am sure. Then I think about where my opponent will go. If this is clear (because it is forced or there is only one more big move or some such) then I think about my next move after his. And so on... I do this until I see a branching point where I'm not sure what the next best move is. Frequently this is only a few moves deep. Sometimes (e.g., in ladders or other obvious tactical sequences) it is many moves deep. Regardless, I'm not moving until I've read what I can, up to the move where I become uncertain. Often times my opponent doesn't make the move(s) I felt certain (s)he would and this sometimes presents a golden opportunity to take the lead or sometimes it reveals that I was wrong in my thinking. There is always a temptation to think "why should I read so far ahead when my opponent seems never to play what I read anyway." This is a self-indulgent justification for not reading. It is always better to look ahead, regardless of what happens in actual play. You'll be better equiped to handle deviations from your hypothesized play that way. Anyway, when I employ this "don't play until uncertain" method, it leads to the funny situation of me sitting and pondering for longer than seems necessary (because my next move may be forced or obvious). My opponent is left wondering what on earth I could be thinking about. Sometimes I tell my opponent I'm doing this so they don't get annoyed. Anyway, I see a lot more of the game ahead of time and this is a very good thing. --SnotNose
Warning: This might constitute as cheating.
8) Write Down Your Move Before You Play It. This is a tried-and-tested technique favoured by chessplayers, and it works. If you are making a game record, write your move onto it before you play it. The only drawback is that this is quite significantly time-consuming, which is a problem given the comparatively short time limits in go tournaments (in chess tournaments, you typically get an hour and a half for just 40 moves). My variation is to mentally commit to the move prior to removing the stone from the bowl to play.
 When I meet a speed merchant on a go server these days, I no longer feel intimidated but play slowly and gleefully anticipate his mistakes.