Losing weight is a satisfying accomplishment both for health reasons and for appearance's sake, but you may find yourself with saggy arms and legs now that the fat beneath your skin is gone. While there's nothing you can do about loose skin outside of cosmetic surgery, you can take control of the underlying muscle. Building muscle through weight lifting will give your body shape, provide some lift, and make your body feel firmer to the touch.
Perform compound exercises whenever possible. Isolation exercises are effective, but compound exercises, which engage several muscles at once performing more than one joint movement, will grow more overall muscle. Do squats and lunges instead of leg extensions and hamstring curls for a good lower body workout. Do shoulder presses and push-ups for your arms.
Use a combination of free weights and machines. Free weights will force the muscles in your arms and shoulders to work independently of each other, which will help highlight muscle imbalances and correct them. Use machines to safely lift heavy weight that you might not otherwise be able to lift unassisted.
Lift weight that challenges you. You don't want to do endless repetitions; that builds endurance but doesn't build up your muscles. Choose weight that will be difficult for you to lift by the last repetition of a set of 12. If you successfully complete the set, raise the weight by 2 to 5 lbs the next time you work out. Aim to continually challenge yourself.
Set up a workout schedule and stick to it. Do a full body workout every other day or break your workouts into upper and lower body and alternate them so you're resting at least 24 hours between working that part of the body. Listen to your body and rest longer if you need to, but be careful about slacking off. If you don't use muscle it will atrophy and you will lose the toned shape you'd gained.