Here’s a good rule: Get in, get it done, and get on with your life! For strength workouts, aim for an hour (including warm-up and postworkout stretch). For cardio, stick to 30 to 45 minutes. Unless you’re training for an endurance event, that’s all you need. Any cardio over an hour will cause your intensity to drop. You may be working out longer, but the effort will be less effective. Warm up for five to 10 minutes, then do 10 rounds of 30- to 60-second intervals, resting 60 to 90 seconds in between, and then cool down for 10 minutes.

Just like a car, if you try to run on empty, you won’t get very far. The key is to get just enough fuel to maximize your efficiency during your workout without going overboard (hello, indigestion!). Aim for 150 to 200 calories, about 30 to 60 minutes prior to your routine. Then refuel with another 150 to 200 calories after your workout. I make a 300- to 400-calorie shake with two scoops of whey protein and one cup of frozen fruit, blended with skim milk and ice. Drink half before the workout and finish it right after.

Crunches on a stability ball give you a much bigger bang for your buck. The main reason: The ball provides an unstable surface, forcing your muscles to work harder in a number of ways. First, you use your abs in a full range of motion instead of stopping at the floor. Plus, balancing on a ball requires using muscles in your legs and butt to keep you stable. And the more muscles you use, the more calories you burn.

It's likely due to a hydration glitch. When you're dehydrated, blood flow is reduced; that decreases the amount of oxygen that gets to the muscles, which can cause them to stiffen and cramp up. Muscles are activated by electrolyte activity, so if those key nutrients--potassium, calcium, and sodium--are out of whack (like when you sweat them out during exercise), your muscles get signals to contract but not to relax. Sometimes sitting too long can trigger spasms, so a cramp might not hit you until long after your workout, when you're relaxing on the couch. When a cramp attacks, stretch and massage the muscle, and put some heat on it. And if you're not already taking a multivitamin, start--it'll give you those important nutrients.

On the spectrum of healthy, protein bars are in the middle. Whole foods like nuts and fruit are better; because they’re not processed, they have more nutrients and your body has to work harder to break them down (keeping you satisfied longer). But in a pinch, a protein bar can tide you over until your next meal. Just make sure it has no unhealthy hydrogenated oils, at least 10 grams of protein, and less than 10 grams of sugar.

Pushups strengthen the chest muscles behind your breasts, but that won’t necessarily boost the girls. Here’s what will: Add dumbbell rows to your workout, and alternate them with pushups, doing the same number of sets and reps of each. This combo pulls your shoulders back and your rib cage up, and—ta-da!—perks up your breasts along with your posture.