You go to the gym. You lift weights. But so far your muscles aren’t looking any more sculpted than when you started. If you’ve been putting in the work but not seeing results, it’s likely that something is wrong with your workout—not your willpower. Avoid 5 common strength training mistakes with these tips from weight lifting guru Wayne Westcott, PhD, Prevention advisory board member and director of fitness research at Quincy College in Massachusetts, and you’ll be a lot closer to achieving that lean, strong body you’re after.

Mistake #1: You’re ditching the warm up. While it’s tempting to forgo a proper warm up, taking a few minutes for a quick walk or jog on the treadmill will help you get more out of your strength training routine. “A warm up increases the temperature of your muscles and tendons, making them more elastic so you’re less likely to injure yourself,” says Westcott. Besides, you’ll burn a few extra calories too! See more ideas for the right way to warm up.

Mistake #2: You’re using the wrong amount of weight. Too heavy, and you could injure yourself, too little and you’re wasting your time. When you’re new to strength training, start with 1 to 2 sets of 15 to 20 reps using a lighter weight (about 50% of your maximum lift, i.e., the amount of weight you can lift once). As you become stronger, graduate to 2 or 3 sets of 10 to 15 reps with heavier weight (60 to 75% of your max lift).

Mistake #3: Your form is sloppy. Lifting too quickly and using momentum are two common culprits that can lead to injury and make your workout less effective. In fact, slower is better when it comes to weight lifting. “Moving slowly actually allows you to produce more muscle force, without putting extra stress on your joints. If you’re using inertia, the weight pretty much carries itself, so you’re not getting as good of a workout,” says Westcott. (Weight-lifting isn't the only exercise where bad form can hurt you. Check out 10 mistakes you make on the elliptical trainer.)

Mistake #4: Your routine is older than your favorite college sweatshirt. It’s easy to fall into a workout rut, doing the same few moves over and over. Swapping in a few new moves every few weeks will help you avoid plateaus. “Your muscles adapt to moves, and you no longer see the same gains in strength after a few weeks,” says Westcott. “Even changing up your workout slightly—say by swapping your tried-and-true bench press with an incline press—will shock your muscles and speed results.” Want to really switch it up? Ditch your dumbbells for something new, like kettlebells or sandbells.

Mistake #5: You’re standing still between sets. If you normally rest between sets, you’re missing out on the calorie-burn boost that’s found in adding mini cardio bursts, like a 2-5 minute jog on the treadmill or jumping rope. You’ll burn more calories, and you can cross off both your cardio and strength training in one 30- to 45-minute workout, says Westcott.

9 Exercises for a Better Butt (at Work!)

Get Stronger Abs With Yoga

Are You Addicted to Exercise?