Ok, i have contemplated this and came up with a foolproof plan. One that even if you get caught, you won't go to jail.
First, find a way to kill her w/o leaving any evidence. That's the tough part. I recommend some drugs grind up and slipped into her water when she's not home. It does require you to break into her house though. I would recommend some kind of drugs that would kill her over time, nothing that would show up on the autopsy report. I don't know of any drugs like that, perhaps you should contact your local pharmacist.
Now, after she is dead and buried and you offer your condolences to her family go grab two friends and 3 shovels. Go dig up her grave and you and your friend can all have *** with the corpse. Be sensible and use a condom though.