According to the “overload principle,” for muscles to become stronger, they have to be challenged with a load that’s heavier than what they’re used to. (Women, think about the weight of your handbag—dinky three-pound dumbbells just don’t compare.) Without challenging your muscles, you can’t substantially strengthen or tone them.
Action plan: Choose a weight that you can lift for only 10 to 15 repetitions before losing good form—trainers call this “working to failure.” (That doesn’t mean your arms should feel like noodles when you’re done, or that you can’t bang out a second set after a minute or two of rest.) The most important thing is that you are progressive and continuously stressing your muscles as you get stronger. When you quit increasing your weights, your muscles quit growing. But don't forget, once you are happy with your body, you must regularly change your routine to maintain that level.
reprinted from January 2012/realsimple.com