Bethesda has a really cool system where if a guard finds bodies, he will ask you about it if you're near by.

Unfortunately, guards rarely discover bodies, NPC's do, but they don't tell the guards.

Guards simply walk past any dead body and don't ask you about it!!

Secondly the only time a guard HAS asked me about it, was for a body that I already paid the bounty for.

Simply put, make guards discover bodies easier, make them always ask about the victim to passers by, make the body dissapear a day after a guard has discovered, thus stopping any reoccuring "arrow in the knee" type business.