I sometimes play fantastically good players, that still make silly tactical mistakes in this game. Straight pool is a very different game than the modern rotation games that most people are used to, and unless you are a phenomenal player who can shoot your way out of anything, there are some mistakes that will seriously stump your growth as a player. IMO the biggest mistake is a failure to look the table over. You should experiment like I outlined in my first post in practice, but in serious gameplay, these points will help you avoid "beating yourself".
1. Look at the balls. Walk around the table. If the stack has been disturbed in any way, look at it, there might be dead balls there.
2. If you have options, do not force anything. Shoot the natural shot. Nothing worse than missing on an open table. If a shot is questionable, do not shoot it.
3. If you have no viable options, TAKE A FOUL. There is absolutely no shame in taking a foul. I honestly have no idea why some people will simply refuse to take fouls in this game. It's a great mystery to me.
4. Don't get the cueball on the rail. A huge number of failed runouts can be traced back to ending up on the rail, straight in or worst of all: both. Usually this starts a chain reaction that eventually leads to getting completely out of line. When your intended object ball is on a rail, you should take great care to avoid having the cueball end up on or close to the same rail. No shot is so easy, that shooting from a rail can't make it difficult.
5. If it is at all possible, keep the cueball above the stack. If you are breaking the stack from underneath, learn the appropriate spin on the cueball to get above it again. You are going to mss those long shots in the top pockets every now and then and it's extremely easy to get snookered or otherwise out of line when you are below the stack.
6. An elaboration on a previous point: If you get into a position where you can either shoot one tricky position shot to get on a trouble ball, or two easy ones, do the latter. It's not supposed to look difficult when you play this game, you know.
7. Don't disturb balls that will go in a pocket, unless getting on them is impossibly difficult. One of the easiest ways to get stuck in this game is to go into a "loose" cluster where the balls are slightly apart. The first ball you hit will take a lot of speed off the ball, and you'll end up freezing to the second or third ball. Usually this problem can be solved much more easily by getting into position on one of the balls, shooting it in and thus clearing the way for the others.
If you have to go into a loose cluster use top or bottom spin to "motor" through, or have a back up plan if you do get stuck to a ball.
8. Combinations and banks have their place, but know your limitations. You are not going to go far in this game shooting 50/50 shots.
9. Take chances at appropriate times. You should take a chance only when the payoff is worth it. Consider the score as well as the make percentage/potential yield of the shot. If your opponent is a threat to runout the game from any deficit, the score might not be as big of an issue, but even then it should be at least briefly considered.
10. When you have a cluster with balls frozen together, LOOK at where the balls are "pointing". Let's say I have a 3 ball cluster where two of the balls form a combination line pointing straight to a rail, and I allready have a ball on that rail, it would make sense to mind the speed of my cueball, so that I don't tie up those two objectballs when I go into the cluster.
11. Think about wether you can reach the shot or not. Stretching on shots will have an impact on accuracy and speed control. If you have to shoot a stretch shot, or a shot with the rest, be realistic about what you can do with the cueball.