I think the first trick is identifying your blind spots. What was the tactical motif you didn't see. Is it something you always miss? Do you tend to see it when it's available to you and not to your opponent?
The important question is why you missed it -- only from there can you begin to patch those holes. It's a long road, but worth it.
I can understand making a mistake or an error in judgement but outrageous blunders in turn-based chess blows me away. I just dropped a piece (Bishop) after studying the position for several minutes. It was after a combination but a simple one. I really don't know how to prevent this. Maybe I just have to keep trying and accept blunders as part of the game.
Any words of consolation or, better yet, wisdom?