Except for one-sided exercises such as side bends and one-legged calf raises, you must focus on symmetrical form. Losing the groove and making an exercise even just slightly asymmetrical greatly increases the risk of injury, due to the fact that taking more of the stress on one side of your body than the other during the squat or bench press.
Do not let the bar slope to one side during barbell work. Keep it level at all times. Both hands must move in perfect unison, in both the horizontal and vertical planes. For example, in barbell pressing, one hand should neither be above nor in front of the other. For standing and seated exercises, the use of a mirror will help you to find and correct asymmetrical form.
A critical factor behind symmetrical lifting is perfect hand and foot positioning. If one hand is placed farther from the center of the bar than the other hand, or if one foot is positioned differently to the other, you will have set yourself up for asymmetrical lifting.
Before any set, as noted earlier, check that you have loaded the bar correctly. If you loaded one end of the bar with more weight than the other, you will inevitably lift asymmetrically. A substantial weight difference will be noticeable during the first rep of a set. Then the bar can be set down or racked, and the loading corrected. A bar that is only slightly lopsided may not be detectable as such and will lead to asymmetrical lifting and perhaps injury. If you lift on a surface that is not perfectly horizontal, it is certain you will lift asymmetrically.