The best ways to prevent improper form are to determine the appropriate intensity, tempo and posture for the specific exercise that fits the individual's goal.
In regard to posture, the best "trick" is to think of yourself as a stick figure, straight and angular. Your head is back above your shoulder, not forward, tilted or turned to look at yourself in the mirror. Your shoulders are back rather than protracted (rounded forward). Your abs and glutes are activated in every motion to stabilize your body. Your legs are in line with your hips (unless specified in specific exercises). Your knees are lose, not hyper-extended (locked out). Your feet are pointed straight ahead, not turned out.
During your reps, you want to continue this "angular" thinking. Many injuries occur when joints are extended beyond certain points in the range of motion. Take a chest press, for example. When bringing the weight back toward the body, many people will take it (barbell, dumbbell, cable) all the way to the chest, causing the shoulder and elbow to go beyond 90 degrees. This over extension with pressure damages the shoulder and sends you to rehab to repair your rotator cuff.
The goal is to keep this solid posture in every rep. If you find yourself swinging in a bicep curl, drop your weight so you can maintain good form. It's always better to workout effectively rather than getting injured. An injury will not only hold you back because of the time it takes to heal, but also the time it takes to "re-program" your body to work efficiently due to the alteration of the kinetic chain in the injury cycle.
Remember this checklist before you start your lift:
- Head back over your shoulders
- Shoulders pulled back
- Abs tight
- Glutes tight
- Legs even with hips
- Knees lose
- Feet pointed straight ahead