(Note - this was originally a blog entry, but I decided this expanded version was worthy of a Tip of the Week.)
When players play poorly, relative to their normal level of play, they usually attribute it by just saying they played poorly. That's circular reasoning - they played poorly because they played poorly? Actually, there's always a reason when you play poorly. And the reason is almost always mental.
I cannot emphasize this enough. Unless something is physical wrong with you or your equipment, or you are seriously out of practice, playing bad is always mental. Period. It's a simple concept that many don't get.
Is there something physically wrong with you? Don't say you are old or out of shape; these are constants that limit your playing level, but they don't make you play bad, relative to your normal level, since dealing with these factors is part of your normal level. No, something is physically wrong with you only if it's something that's not normal. Other common chronic problems that might lower your level include knee problems, back problems, sore arm, etc. But again, these do not make you play poorly; they simply lower your overall level of play. Maybe you were just tired? Well, that's a reason to play poorly; get more rest!
Is there something wrong with your equipment? This can happen, but it's rare. Usually it's your own fault. Is your rubber surface dirty and so not grabby as usual? Clean it. Is it getting old and so is losing its bounce? Get new sponge. (If you can't afford new sponge, then it's a chronic problem.) Is it humid? Keep a towel just for your racket and ball so you can keep them dry. I'll also include here other factors in the equipment category. I'm used to playing in near perfect conditions, and so when I play on slippery floors, on brown wood floors with orange balls (where the floor and ball are similarly colored so I lose the ball in the background), or in bad lighting, I play poorly. Is this an equipment problem or a chronic problem? This might classify as both. The fact that I don't play well in poor conditions is a chronic problem that lowers my overall level of play, but only in certain circumstances.
Are you out of practice? Whose fault is that? Solution: Go practice. If you don't have time to practice, then it's a chronic problem that simply lowers your overall level of play.
So if nothing is affecting your play physically, and you can't blame it on your equipment, and you are in practice, what can you blame your poor play on? Yes, it's all mental. There are so many ways a player can talk himself out of playing well. The most common is by harping on the poor shots instead of the good ones. Harp on the good ones, and you'll likely begin to emulate them. Harp on the bad ones, and that's what you'll continue to do since that's what you're thinking about, and the brain (i.e. the subconscious) tends to mimic what it is thinking about. Or perhaps you got nervous? Yep, that's all in your head. Or you got defensive when you had a lead? Another mental problem. Or you just weren't up to playing? Yep, that's mental.
I could go on and on, but the conclusion over and over is the same. When you play poorly, it's almost always mental. And guess what? With practice, you can get control over the mental aspect. You have to choose to do so, but once you do, you'll rarely have a bad day again. Here are some resources on sports psychology.