It is generally accepted that you get the fastest xp by using the Rapid style

Experience points you receive towards your ranged skill depend on which of the three attacking styles you are using, shown below. With Accurate or Rapid styles, for each damage point you inflict you will receive martialempires gold four ranged xp and 1.33xp hit point’s xp.

However for Long-range, you will receive two ranged xp, two defence xp points and 1.33 hit points’ xp. It is up to you the style you want to train with, however it is generally accepted that you get the fastest xp by using the Rapid style.

A quick example is that if you hit a 12 on a Hill giant using the accurate style, you would receive 48 ranged xp and 16 hits xp.

You can either attack up close or you can use an obstacle to stop the monster from reaching you. This is a useful tactic to avoid damage and can be seen in the picture below.

However, be aware that some monsters have their own ranged weapons and will attack you from a distance as well. An important point is that some of your arrows fall next to the monster and you can pick them up to use them again.