First, you might want to exfoliate. Exfoliating rids you of dead skin and prepares the area to be shaved. You don't have to do this every time you shave, but it's a good idea to do it if you haven't shaved in a while. To exfoliate, use a loofah or a gentle exfoliating body scrub.
Be sure your skin has had a good soak. Many women prefer to shave at the end of a shower or bath so that their skin has had a chance to enjoy the effects of the water.
Start with a nice sharp razor. This is especially important if you have a formidable amount of hair to get rid of. When your razor is sharp, you can shave with a light touch and avoid skin irritation.
For legs, start at your ankle and shave in long, gentle strokes in an upward motion. Make your way around the leg until all areas are done. You can run a hand quickly over your leg to make sure there are no hairy spots left. Don't forget your feet. Women often don't want to think about the possibility of stray hairs on feet and toes, but they definitely happen. Always be gentle when shaving this area; the skin of your feet is thin and can easily get nicked.
Shaving your underarms can be a bit tricky because the hair there grows in a pretty random, wild fashion. You have to make sure to shave thoroughly and in all directions. To make shaving your underarms easier, pull the skin taut by raising the arm high.