One thing I learned playing correspondence chess: don't make your moves late at night when you're mind's starting to shut down.  I had a streak recently where I had some really dumb losses and my rating dropped nearly 200 points, all because I was making moves in a soporific stupor way beyond midnight.  I've still not recovered completely, ratings-wise, but at least now I'm making moves during the day with plenty of caffeine flowing through my system.