Motive: You have to eliminate motive. For a revenge killing, there should be a grace period between the incident which starts the character thinking murderous thoughts and the murder itself. Maybe time is of the essence, so a workaround would be to feign friendship, so that people would not connect the emnity which has arisen with the later murder. The other neat thing about planned murder is the opportunity to drop other people in the shit. especially if grudges exist elsewhere - it's killing two birds with one stone. Kill the first guy and set up the second guy.
Opportunity: Alibi is important is you want to get off the hook completely. It may seem like SF, but you can rig up a lethal device with minimal knowledge of electronics or explosives - especially since the internet is now all-pervasive. Are you planning on using something low-key or a flashier demise for the murdered character?
Ability: A murder which is obviously above the ability of the killer would be a good way to keep prying investigators off track. Reading up on the technique used, then feigning ignorance of the basic requirements is a simple way of 'proving' innocence, but there are a few ways I could see this being done that would be very quick for a killer to pick up.
Forensics: If you're gonna kill a person in their home you better have a previous meeting scheduled with them to handwave away any fingerprints or hairs found at the scene of the crime. During the commission of the murder the usual precautions would need to be taken, but having reason to be at a location doesn't hurt.
I haven't really thought of murdering anyone. Honest.
