Over plucking is bad! One because you end up with no brows (sometimes they do not grow back) and two because you end up completely changing your face shape. It's also true of not keeping them maintained!
If your really stuck with your brows the first thing I would suggest is to get yourself down to a decent brow bar or salon. Explain what you are finding difficult and let them re-shape them for you.
I have dark eyebrows, which always grow thick if left, so prefer to leave them as natural as possible. Otherwise they end up looking like caterpillars! I do like to add a little powder to fill in any little gaps. Of course if you do have dark hair and want to thicken them up a bit I'm not suggesting you will end up with caterpillar brows.
If you have fair hair you may find you lose them a bit. Again I would use a little powder to fill them in and build up the colour. If they need thickening up but you don't feel confident with a powder invest in a brow gel with fibres. Much easier to build up the brows.
A general rule is that the end of the brow should be in line, diagonally, with the end of your nose and you should avoid taking too much hair away from above your brow. Also only take away some hair that is above the inner corner of your eye otherwise you end up with a massive gap between your eye brows!
For more information on filling them in check out my basic eyebrow shape blog.