Mostly everyone hates leveling up prayer. It takes so long! Well here's a members only tactic that makes it go a lot faster. This tactic is using the ectofungus.
The ectofungus is found on the world map right by Port Phaysmaty in the Northeastern corner. The only way to get here is if you have done the Priest in Peril quest. The quest Ghost's Ahoy also helps as it gives you a way to teleport straight to the ectofungus and a toll free entrance to the port since there is a bank there. Anyway here's how it works.

First off you need bones. Different kinds of bones give different xp. Big Bones (found by killing any Giant monster) are popular choices and maybe Dragon Bones if you are high leveled in combat. I wouls get a minimum of 100 (since you will be traveling a long way to get to the ectofungus)

You need an equivalent amount of pots and buckets. (As many pots and buckets as bones. Ratio:1-1-1)

Take 9 of each item and go to the ectofungus. You will see Ghost Disciples walking around but ignore them right now. Go down the trapdoor and run along the circular path descending stairs (agility shortcut available) until you get to a pool of green slime.

Fill your buckets with this slime and return to ground level. Now go upstairs to a machine called the bone grinder.