Consistency tips:
1.Footwork. Everyone needs to get into position to hit good shots consistently.
2. Good, repeatable technique. Flaws in technique can cause errors, obviously.
3. A good and smooth swing path. All good players have a solid swing path they can go through to keep the ball in. Important elements of a good swing path is going from low to high, and going through contact. I personally also like swingpaths that go up, through the ball, and across then followthrough.
4. Timing/point of contact. Contact should be in front of you. Timing your swing to meet the ball in front of you is a key element in being consistent and hitting hard too. You should try to move to a ball so that you hit the ball at your optimal contact height (usually around hip level or a bit higher).
5. Leading your stroke with your core/legs. This is more of a personal tip I use. Your legs and core rotate the least, compared to your shoulder and arms. Leading your stroke with it helps me keep my stroke under control, yet hit hard and efficient because of the use of the whole body.
6. Topspin. Hitting topspin gives you more margin for error, although too much brushing up isn't the best way to hit it. If you like to hit the slow loopers, then try to hit through the ball more and get a heavy topspin ball instead to improve your tennis.
7. Keep your head still. Jerking your head around will cause you to shank/mis hit tons of balls. The prefered method would be to keep your eye on contact like federer does, although not necessary.
8. Use of the nondominant arm (applies to everything but the 2hbh). On serve, 1hbh, slice, forehand, and volleys the pro's use their nondominant arm. It actually helps a lot. The nondominant arm helps keep balance, hit through the ball, transfer your weight, control your body movement so you don't open up too early on the forehand, and space yourself from the ball.
9. Staying loose. Tightening up can cause everyone, including the pro's to miss. On the other hand being too loose and spraying balls everywhere isn't good either. Stay loose to keep your form and swing smoothly.
10. Accelerate through contact. Slowing down your racket head speed before you hit the ball will cause you to hit uncontrollable shots. Also, maintaining the same swing speed throughout will cause you to not hit with enough spin. No matter how fast or slow you swing, you need to be accelerating through contact.