I have put a lot of time and thought into this exact topic. I feel now that my percentage on a straight shot is near 100. These are the things that have helped me over the years:
1) You need the correct visual to see the shot the right way. I would recommend giving a call to Geno on here...he has put a lot of study into seeing the shot right. A friend of mine, actually the manager of Snookers in Providence, RI helped my out *tremendously*. He has these little black stickers. They look like the number 8, with two circles exactly the size of pool balls. The idea is you put the sticker down on the table with the little arrow aiming straight into the pocket. You place the object ball on the front "ball" of the sticker. The back ball is like a ghost ball, with a little red dot in the center. Now, the strange part is, you take the thin white backing to the sticker and stand it up against the back of the object ball, such that you cannot see the object ball when you are down to shoot. All you can see is the rear ghost ball with the red dot in the center. You then practice shooting the cue ball at the red dot. You know you did it when the object ball goes straight in. This obviously will be nothing like most people aim, but that isn't the point. The point is to shoot perfectly straight *AT* the thing you are trying to shoot at. Most people just don't see "straight" correctly. I can't tell you how much this helped me.
2) Alignment: pretty self explanatory. I used to set up a shot that was straight in diagonally across the table, with the OB 2 diamonds up from the end rail and 1 diamond over from the side rail, and the CB the same on the other corner. The goal is to shoot the ball in and draw back into the corner pocket near me, *nothing but net*!! Meaning you can't even touch the facings of the pocket. I've done that shot 100's of times. I noticed that 2 grooves appeared on my table...one perfectly straight, and one that veered off to the left about 1/2 inch. Clearly on those left shots, my alignment was off. So you really gotta fix this. I simple test is to set up a straight in, get down to shoot, and close your eyes. Relax your arm 100% and just stroke. If the ball is going in, you probably have good alignment. If not, you will be fighting yourself all the time. Get a competent instructor, because its really hard to see yourself. Or video tape yourself from head on or directly behind.
3) Do the drill I just mentioned. A LOT! You really need to learn how to aim thru the cue ball at the center of the object ball. Then just connect them. Gotta be free and loose. But to be honest, I have tightened up my grip a bit, sort of "loading" my wrist...I don't find it hurts my accuracy one bit. If anything, it might be better.The main thing is not to clench when you shoot.
4) Find a mirror that you can stand up at with your cue. Bend down to shoot as if the cue ball is right at the mirror surface. Look only at the cue tip in the mirror coming at your cue tip in reality. You want to align yourself so that it looks like if you stroked thru the mirror tip would go directly straight into your real tip. In other words the two cues should form a perfectly straight line. You will have to adjust where on your face you hold the cue in order for it to look like this. Now, when it looks perfectly straight, see where the cue sits on your face. *This* is your center of vision. You should make a real point to place this part of your face very precisely on the line of the shot. This way, you will *see* what *straight* is. This is so huge I can't even say.
I'm sure there is more, but I think if you try these things you will improve your accuracy on all shots, but especially straight in.
Hope it helps! Good luck and report back!
KMRUNOUT