An excellent source of Prayer experience is Dragon Killing. Their bones provide a lot of experience and don't require extremely high levels of stats. To take out Blue Dragons below the Taverley (in Taverley Dungeon) you do not require a lot of resources as they can be killed from a safe location. If you're looking to accomplish this, remember to do it with Magic since they are insensitive to this specific style of fighting. Utilize giant eggs, rocks and columns to get Dragon stuck, and then end them by casting your spells.
Also, you may kill Green Dragons north of the Graveyard in the Wilderness. You just need to bring the Anti-Dragon Shield, food and equipment. Do not bring anything that is precious so you won't lose any money from Player Killers. It's also worth considering farming the area for Void Knight Equipment as it isn't lost by falling victim to it while in low Wilderness. It's an ideal outfit to use for Dragon Killing and a safe one for wearing in this zone.
All bones that you collect must be sacrificed at the Chaos Altar, but it can be a risky method to take anything else than the bones you'd like to sacrifice. This is among more Player Active zones on Runescape located on the 38th level in Wilderness.