Are we talking about a single person? or multiple/serial type?
A single person, the best way is NOT to get caught. It is best to make it look natural.
this of course takes quite a bit of work. You would have to know your target and habits. People can die from a lot of things. It doesn't take much to kill a person. I mean you can drown someone with a teaspoon of water.
Serial type?
never take a souvenir.
never leave a "marker"
never do it the same way
never follow a pattern