A good way is to go into a dark area without a light source (like through the hole in the Lumbridge Castle Cellar which leads to Lumbridge Swamp Dungeon) and you will be hit with RAPID 1s.
Another way is to rapidly drink Zamorak Brews, but this only damages you so many times in a few minutes.
The second way is better if you just want to get your hitpoints LOW, such as for using Dharok's.
Although some reccomend going to the wilderness and trying to run into a Revenant, this is NOT a good idea, as Revenants are not found in any one definate place.
Another possibility is to try to kill a Slayer creature that you don't have the level/equipment to kill it with. It will attack you and most likely kill you.
And don't attack the monster that's attacking you.