I agree with Sihaya - a lot depends on the person doing it. Just as there are great hair stylists and bad ones, I think some people definitely have an eye for brows. I have really thick brows that grow very quickly, and I found a fantastic person at Macy's Benefit Brow Bar. Costs $20, I go about every five weeks, and she's great. She just has an eye for the right shape - you shouldn't really change the shape of your brows - you want someone who will just make your own shape tidy. I used to do tweezing only, but now I wax and it does last longer. (Threading probably lasts longer, too) I went to another woman at the Benefit Brow Bar, and she was great too - I just happen to prefer the one I go to now.
Try to resist the urge to do too much yourself between appointments. As Sihaya said, just one or two hairs can affect the shape. I tweeze between, and if I have strays below, but I avoid anything too close to the actual "shape."
I have a slanted Tweezerman, but I'm very rough on them. Mine get dull quickly, and I find I have to replace them once a year. (You can supposedly send them in to get sharpened, but I've never done that. I keep meaning to!)
I only get a pedicure a few times a year, but I do treat myself to brow shaping regularly - on me it makes a huge difference.