Before starting a resistance training workout, you should always perform a few warm up sets. This is for two reasons, really: 1) It will prepare your muscles for the workload that is about to be placed on them, and 2) It raises your core body temperature, helping to get the blood flowing.

To warm up properly, use a much lighter weight than you normally would to perform a specific exercise. For example, if you are doing the bench press, and you usually lift 185 pounds for sets, warm up with a set of ten reps at 135 pounds. Then, you may also want to perform a second warm up set, only this time using a somewhat heavier weight than the first warm up set (maybe add an additional 10-pound plate to each side of the bar). Once finished with that set, you should be sufficiently warmed up and ready to jump into your first real set.

In addition to warm up sets, it's a good idea to perform at least a 5-minute cardio warm-up on a treadmill, bike, or elliptical machine. This, too, will help get your body loose and ready for your resistance training workout.