This is a very quick and easy tip.

Always position your finger close to the fret, but not quite touching it. Look at the photos below that demonstrate bad and good finger positioning on the string:
Here the finger is incorrectly positioned in-between the two fret:


Here the finger is perfectly positioned, very close to the fret but not quite touching:


Here is the finger incorrectly positioned too far from the fret:


And here is the finger incorrectly positioned on top of the fret: