I have problems with my game, im not a great player but im not bad either but it frustrates me when i go to hit balls to a corner to make opponent run for it or to grab the point and i hit outside of the line making it a point for the opponent. I do this frequently and it does my head in, i have tried not to hit as hard whilst hitting from side to side making my opposition run and to try and gain more accuracy instead but it just doesnt seem to reap results because the shots are not powerful enough so when i do put more speed on the shots i hit them out. Whats the best method for making sure they stay in the lines, is there some kind of mental excerise or way to think when hitting shots like this?
Well, first of all, the simplest way to get better at something is to do it over and over again with the intention of learning from your mistakes. What I mean is, pay attention to what you're doing when you're hitting the ball out so that you can fix it. A lot of players practice for consistency but they do it blindly, such as repeating the same motion while expecting a different result.
Now, hitting from corner to corner is a very specific action, but it all boils down to just accuracy and consistency. Since you can already hit from corner to corner and your problem here is not being able to consistently do so, I'll just skip to the consistency part.
Is your form consistent? This leads to another thing, footwork. If you're not getting to the ball in time to prepare, of course your form will suffer. Make sure you're keeping your head fairly still, a lot of head movement will lead to inconsistency in your shots.
Are you following Wardlaw's Directionals? If you don't know what this is, it's pretty much just court positioning and how to play high percentage Tennis. For example, say you're trying to hit from corner to corner and you're at the middle of the baseline and so is your opponent. Your opponent first hits a ball straight at you, you reply with a forehand cross-court toward his right corner (assuming that both you and your opponent are right-handed). Your opponent gets there and returns the ball straight at you. Now, considering that you are still in the middle of the baseline and the ball is coming toward you from your left, and you want to hit it toward the other corner, this automatically becomes a physically more difficult shot to hit. Nonetheless, if you have a decent backhand, it would be a better choice to use over your forehand. A cross-court backhand in this situation is safer than an inside-out forehand.
What you should focus on here is not necessarily more or less pace/power, rather focus on your form and timing. If your form and timing are correct, the ball will usually go in. Remember, the court isn't moving, it's always there. All you have to do is look at the ball as it's coming and make contact with it. You do NOT have to follow the ball with your eyes right after you hit it, keep your head still and down while making contact with the ball. Notice I'm emphasizing head movement a lot since it was my major problem for inconsistency. I used to always follow the ball after I hit it, which causes inconsistency because my head jerks up after a shot.