If you currently stretch before you lift weights, you may want to stop. Research has found that muscle strength performance decreases from 45 to 28 percent when preceded by stretching exercises. Possible reasons may be a delayed neuromotor response that occurs immediately after stretching and an increase in muscle compliance, which may limit the capacity of muscles to produce force. Research also suggests that stretching doesn't protect against injury, or does it diminish muscle soreness following a workout.

I just tell people (I work at a local Planet Fitness part-time, and do some of the training) to walk on the treadmill briskly for about 5-10 minutes to warm up before lifting weights.

I agree. I usually do my cardio at home before my workout, then i walk to the gym and warm up for 3 minutes on the treadmill all before my workout.