There are countless ways people sabotage themselves while strength training, but the most common (at least for women) is not lifting enough weight. Despite research to the contrary it seems women are still worried about bulking up. How about helping let this myth die? In truth, few women posses the genetics or testosterone to build huge muscles. Bodybuilders train in a very intense, specific way, and even then, many resort to steroids.
To reap the benefits (and there are many) of strength training you need to challenge your muscles.
How much should you lift? It all depends on your fitness level and your goals. If your goal is to firm up, gain muscle tone and improve your strength and health, you need to lift enough weight so that by the time you get to 10 or 12 repetitions it should feel like you could do very few more. If you are cranking out 20 reps or more you are probably not getting the results you want. Beginners can stick to one set, but if you have been lifting for a while or are trying to lose weight, aim for two. Research is iffy as to the need for more than two sets. If you are training the whole body at once, two times a week should be enough. You do not have to lift to failure; but if you lift heavy enough you will see results much faster, and you will no longer be wasting your time.