i've been cutting my own glass for a while, and we have half of our workshop dedicated for stained glass (my parents do that in the winter)
Here are a few tips
*make sure the glass is as clean as possible
*buy a good glass cutter that has a handle you fill with glass cutting oil. if you don't have one, make sure you dip your glass cutter in glass cutting oil.
* start at one end, and score the glass with the cutter all the way across the glass. Make sure you don't stop at the end, just drop off the edge of the glass
*cut the glass on something like wood or something not super hard, so when the cutter drops off the edge of the glass, it doesn't dent the cutting wheel
*use CONSISTENT pressure on the glass. Don't push down hard for one part, and light on the rest, as this will mess up the break. Use a little more pressure than what you would when writing with a pen or something. You will hear the cutting wheel score the glass.
*use CONSISTENT speed while cutting. Don't go too fast, because you will probably get off center, and end up with a piece that doesn't fit. Don't go too slow either, because the cutting will be "jerky" and inconsistent. A nice steady speed is what you are aiming for.
*NEVER go back over a score! you have one chance and one chance only to get it right. One score with a nice steady speed and pressure, all the way through from edge to edge will give you the best results.
*if you are cutting a straight line, break the glass down the score by putting the score on the edge of a table or something similar, and apply downward pressure until it snaps. Make sure the score is facing up, and you push down!
*use a straight edge for straight cuts. it will save you a lot of pain!
*practice on scrap pieces before you cut your real piece! Remember, you only have one chance, so don't mess up!
do these things, and you will get a much higher ratio of good cuts to bad cuts. Hope that helps some!