If there were only one exercise you could do to sculpt yourself a pair of lean and sexy arms - besides the squat (as I've mentioned before, for burning body fat and transforming your entire body, nothing else really compares to a squat) then I recommend the push-up.
The push-up is the single most underrated, and under appreciated upper body exercise there is. Done right, it can sculpt your arms, lift and tone your chest (pay attention ladies - before you even consider a breast lift you need to try this exercise first), tighten your stomach, stabilize and strengthen your lower back, and build tremendous strength.
The push-up is also the most poorly executed exercise I see on regular basis. In fact, most women don't even attempt it because they think they could never do even just one. This is nonsense. I have seen so many of my personal clients come to me who couldn't do even one 'girl push-up' when they started, and a couple months later they are doing full 'men push-ups' better than most guys.
So to help show you the correct way to do a push-up, I snapped a couple photos when I was doing one of my 15 minute home circuits the other day (I'll post the full workout at a later date).
1st - Correct Way
This is the proper way to execute a pushup (that thing I am holding onto makes the pushup more difficult by lifting me off the ground a few inches).
My back is pretty straight as I'm pushing up (except for my bubble butt - a result of very tight hips and lots of heavy squats - hey I'm working on it ;-) and my hips aren't drooping down.
2nd - Incorrect Way
In this photo, see how my back is lifting, my hips are drooping towards the floor, and I'm arching backwards? This is really, really bad form, but the way that most people do pushups.
If you can't keep your back straight when doing a pushup, then you MUST switch to a modified pushup or 'girl push-up' until you are strong enough to do at least 3 regular pushups with perfect form (see below).
3rd - Modified Pushup
In this photo, I'm going from my knees, but my back is straight (again, except for my butt sticking out) but my hips aren't drooping, and it's significantly easier than a regular push-up. This is where you'll want to start if you can't yet complete a regular push-up.
To get the most out of the push-up, start by doing three sets of as many reps as you can (with perfect form), three to five times per week. Once you can complete 15 reps of the modified pushup, switch to the regular pushup. This will build strength and muscle tone quickly, and you should be seeing some changes in your upper body within about 2 weeks.