You can improve skill in making bank shots in billiards by studying simple geometry, though most players recommend the tried-and-true method of repeated practice and getting an understanding of how the balls and banks interact with each other through feel. The english on the cue ball and object ball, as well as the speed at which the cue ball is traveling, all have an effect on the angle at which the object ball bounces off the rail.
In a bank shot hit straight on, where the cue ball hits the object ball in the center, the object ball bounces off the rail at the same angle as which it approaches if you hit the cue ball at the correct speed.
Easy Pool Tutor describes in a tutorial an advanced technique called the Equal Extended Rail Distance Method. Draw a line from the object ball perpendicular to the rail intended to be used for the bank (line A), then draw another line from the point line A touches the bank to the intended pocket (line B). Draw a third line, line C, from the object ball to the pocket opposite the intended pocket. The place where lines B and C meet is the place on the rail where the object ball must hit for a successful shot.