Free-to-play Prayer training

Prayer is considered the hardest skill to train in F2P world. There is no specific advantage for any skill to raise prayer quickly, all it does is money and effort. In F2P worlds, raising prayer above level 45 is hard to foresee its advantage as no new abilities can be unlocked. Prayer has also the highest cost per experience point to power level among all the free skills, while others are no more than 10 coins per XP, prayer can get steady above 15 coins per XP.

Beginners
Prayer is relatively straightforward to train with, making it slow and boring to train as comment by many free players. The main source of experience is regular bones which yield 4.5 XP each, and worths each if sold to Grand Exchange. Here is a list of things that can help raise prayer slightly more efficiently.

The Restless Ghost
The restless ghost is a quest that gives 1125 prayer XP, allowing a player to raise prayer from level 1 to 9 upon completion. This has no effect on speeding up the training process at higher levels though.

Respawn places
For players not willing to power level prayer, some places must be familiar with, in order to get quick bone drops:

Lumbridge
Many players just leave bones on the floor after killing monsters in Lumbridge. Bones is often abundant there for players to pick.

Edgeville Dungeon
Edgeville Dungeon inhabits hill giants which is the weakest giants to drop big bones. Many players consider hill giant as a good target for its drops and big bones, thus the dungeon is often crowded. Actually, due to the excessive population there, finding a hill giant to kill is difficult and not recommended for prayer training, though two bones respawn in the dungeon near them.

Al Kharid palace
Low level Al Kharid warriors are often killed for training by players. Bones are often left on floor as well.

Training prayer by killing monsters
Depending on the max hit of a player, different monsters should be trained for prayer experience. A good prayer training monster should be weak enough to be killed in 1-3 hits for the current ability of player.

No requirement
Chickens, cows, goblins, men and women can be killed in Lumbridge. Any fighting methods can be used because they are weak enough to be killed even at low levels.

Combat 41
Dark wizards can be killed as they no longer become aggressive towards the player. Bones on the floor can be also collected.

Combat 51
Skeletons and Zombies can be killed in several dungeons. They generally have low defense and hitpoint levels. Minotaurs can be killed in Stronghold of Security, because it is popular with rangers and being weak, bone drops is available in busy worlds.

Combat 60
Hill giants in Edgeville dungeon drop big bones, and two bone respawns are available there.

Combat 60, Prayer 37
Monks of Zamorak can be fought and once they are all taken out, the Wine of Zamorak can be taken without any trouble. The altar there is ideal for recharging the prayer for Protect from magic.

Power leveling by purchasing bones
There is only one way to power level prayer: burying regular bones bought from Grand Exchange.. To power level prayer, coins per XP is lost. To attain level 45 from zero experience merely by power leveling, 13670 bones are needed, costing coins.