Smacky's Guide To Magic, Section 1-2: Divine prayers

Training divine prayers

Experience gained via invoking prayers goes to the divine prayers skill, and as your level in this skill increases you will be able to invoke more powerful prayers.

You will only get experience for killing a mob with a prayer. Therefore, non-offensive prayers will not train divine prayers.

Prayer power and cost

The precise number of individual prayers you can invoke is determined by your current reserves of grace. Grace is affected by your level in divine prayers and wisdom.

So your grace increases as you level divine prayers. However, the cost in grace of prayers also increases (as does their effectiveness, eg, damage done) as you increase your level in this skill. Therefore there is a theoretical trade-off in levelling divine prayers.

Some effort has been spent balancing the game so in fact the trade-off is not something with which to be overly concerned -- the practical issue to keep in mind is that as your maximum grace increases it will take longer and longer to regenerate.