If you're are trying to draw straight lines with a mouse, then it's best to do as nogood said and use click+shift+click. Other wise it's nearly impossible for someone to draw a straight line with a mouse.
If you're using a tablet however, drawing using your whole arm (elbow and shoulder) and locking your wrist will help you gain more control over your stylus and thus creating a nice wide range over your canvas, making it easier to make straight line strokes.