The number of women who insist on having their eyebrows tended to by The Professionals kind of amazes me, because it's really not that difficult to do a great job on your brows at home. Unlike attempting to wax your own lady garden or pierce your own ears, this is a job that isn't necessarily best left to a specialist. Yes, there are lots of horror stories about people ending up with chronically wonky or too-thin eyebrows having attempted to DIY, but as long as you take it handy, avoid magnifying mirrors, and don't lose the run of yourself altogether it's dead easy to get and maintain a fabilis shape.
For most people, the best time to tweeze is after a shower or bath as the pores will be open and hair can come out more easily. If you find tweezing painful, you can use ice to numb the area temporarily.
To determine where your eyebrows should begin and end, hold a thin pencil or brush vertically against your nose (I use my Smashbox Arced Liner Brush for this because it's proper skinny). Anything visible between the side of the pencil and your other brow has got to go. Then hold the pencil diagonally so that it extends from the corner of your nose to the outer corner of your eye - again, anything on the outside of the pencil is excess.
Brushing your eyebrows upwards will help show you the natural line of your brows and make it easier to identify where stray hairs are lurking, and you might find it helpful to use a white eyeliner pencil (or a dark pencil if you're fair haired) to mark the hairs you intend to remove. That way you'll be able to get a good idea of the shape you'll be left with before you even pick up your tweezers and avoid the dreaded tadpole effect (chunky, sometimes rounded inner eyebrow tapering to too-skinny ends).
Use good quality sharp tweezers that have thin tips and a strong grip, like a Tweezerman slant for precision or one of Aphrodite's recent discoveries, and do the job in natural light. Hold skin taught and grasp hair as close to the skin as possible with your tweezers, then pull out in one fluid movement in the direction of hair growth. Remove one hair at a time. Shape your eyebrows by plucking from underneath to preserve their natural arch: the highest point of your arch should be directly above the outer corner of your pupil. You can use your trusty pencil one last time to work out where this spot is if it's difficult to see without an aid, just hold your pencil against the edge of your nostril up to the brow across the outer edge of your pupil and mark the point with a dot of eyeliner pencil.
The most important thing is to take your time. To ensure symmetrical results, tweeze just a few hairs on one eyebrow and then move across to your other eyebrow to duplicate the process, and be sure to step back from the mirror frequently to assess your progress. It's also a good idea to stop once they start looking done: a few over-zealous plucks is all it takes to have brows looking way too thin and you can always come back in 10 or 15 minutes to see if there is really any more to be done at them. Don't apply make-up to the area for 30 minutes after tweezing your eyebrows, as your pores are still open.
Alternatively, if that all sounds too scary, go to a pro for the initial shaping and then you've just to worry about the upkeep! Just remember to eye up your therapist's brows before committing yourself into her hands...