ways I do that (legally): Really mistreat someone (like your wife), hopefully they'll start a plot to kill you.
Put yourself in every battle you do, try to lose them ;)
In a more 'cheaty' way, you probably can load the game as one of your counts/dukes and select the plot to kill the liege :) then go back to yourself and wait until they succeed
gaming.stackexchange.com/users/16788/Rodolfo
One of the most reliable ways is to try and assassinate someone with very high intrigue.
They will discover your assassin, and then they are very likely to send an assassin after you.
If they fail, or don't send an assassin, just try again.
Conveniently, after you die the reputation hit from assassinations disappear.
gaming.stackexchange.com/users/26042/Lawton
Just keep throwing him in harms way.
Hunts, get into as many fights as possible, piss off the natives, go crusading at every chance.
That should up the odds nicely.
There's gamey ways to do it like Rodolfo stated, but who would do that?
;)
gaming.stackexchange.com/users/8083/canadiancreed
You can also commit suicide.
This option is available in your decision tab, if your character has the "depressed" trait.
I'm uncertain when this was introduced; I don't think it was possible in 1.0.
gaming.stackexchange.com/users/25281/Nix
Another way to get your heir on the throne is for your current character to be forced to give up the throne as a result of surrendering in an excommunication war or a war to dispose of a tyrannical liege.
In the case of the latter here are some ways to get your vassals motivated to stand up against your rule:
revoke their titles imprison
them execute said prisoners
I'm not sure
what the odds are they won't start a war for independence as opposed to a war to depose of their liege, but if your current ruler is a bit of a douche to start with it's another in-character way to get what you're after.
gaming.stackexchange.com/users/27124/Eric
The current ruler of my dynasty is a bit of an idiot. His heir shows much more promise; is there a way to get rid of my current ruler?
I dislike the current head of my dynasty, can I arrange an accident for him?
gaming.stackexchange.com
<crusader-kings-2>
gaming.stackexchange.com/questions/52687
gaming.stackexchange.com/users/11240/Xenox
You can put yourself in every battle, mistreat someone so they plot to kill you or load the game as one of the counts and select plot to kill liege. It is indeed possible to arrange an accident for a character and the community suggests few ways to do so.
The community provides few options by utilizing game mechanics.