As a general rule for your practice strokes, look at the cue ball as you stroke towards it and stop right next to it, and you look at the contact point on the object ball throughout your back swing. Within your routine, you may choose to look at either the cue ball or object ball for one or more strokes in order to really key in on your accuracy. 

The reason you look at the cue ball when your tip is extended, is to prepare for the accurate cueing of the ball. That is also why you stroke directly towards the exact spot you want to strike. Remember, it is like a dress rehearsal for your stroke, so pay attention. 

The reason you look at the object ball when you start your back swing is because this is what you should do right before your final stroke. You need ample time to focus on the contact point (which is what you should be looking at on 99% of your shots) before you strike the cue ball. On these practice strokes you are verifying the contact point and making sure you are lined up to hit it. 

Now, it is up to you to put these ingredients together in a way that suits your own style, in particular the number and speed of your strokes. It might be a good idea to model your favorite top player’s eye pattern. 

Once you develop a system that works for you, be sure to use it in all situations. Sometimes the pressure of a match disrupts your pattern and throws you off a bit. Try to get back into you pattern as soon as possible. Your eyes are amazing instruments and they will let you know if everything is on line, try using them in coordination with your cue stick.