Okay first of all let me say that I'm not planning on killing anybody. I have been debating this with my brother for like ever and it's just for fun. What would you do to kill someone leaving no evidence and never getting caught?
Easy. Just target someone who you have absolutely no association with. No motive that'll point the finger at you. Obviously, don't get seen. You can leave fingerprints so long as you're not any fingerprint systems and stay that way. You can leave all the dna evidence you want too. They'll never know to compare it to you. The most "successful" serial killers have no pattern. Just look at that guy a while back... who traveled the rails and just killed randomly. He got away with it for decades.
With every crime scene there is the transfer of evidence. Anyone coming in or out of the scene is guaranteed to pick something up and leave something behind. latex clothing will help prevent hairs and fabrics from being transfered. You're feet leave prints and thus must be altered. this can be done by simply wearing shoes 2 sizes to big or to small. A persons footprint can determine a persons hight and weight. BURN everything that was worn at the scene of the crime.
Body decay is a factor that can be over come by either increasing or decreasing the temperature of the room for at least 24 hours. However, this can be discovered by checking the electric usage. Any spike in use can help determine an accurate time of death (TOD). To over come this simply move the body from the original crime scene to a new location. Don't bother trying to make this new location appear to be the original crime scene as it will be quickly proven otherwise. Painters tarp is recommended for transferring the body as it will help reduce the transfer of evidence. Take the tarp with you and burn it.
Strangulation is an effective way to avoid detection if done right. Don't use your hands they will leave bruising. Instead use a rope/cord. If you are taller than the victim pull down on the rope to make you appear shorter, If you are shorter, Raise the your arms up to appear taller. The bruising left by the rope can determine a persons hight and anything that can conceal any physical attributes about you is crucial.
You follow them at a distance for several days and learn their basic routine. On the appointed day, you follow them on a motorcycle along their most rural routine route. When they pull up to a stop light and there's no one else within sight, you pull slowly up beside them, give em two in the noggin, and drive away. Unless you know them fairly well, you'll never even be so much as questioned in the case.
I would caution you to not leave fingerprints or DNA just in case. You might be in some system you don't know about (I recently found out my parents had me fingerprinted as a child).
Also, attempt to establish an alibi for the time in question. Since that's difficult, seeing as you were really off committing murder, I would say to put the body somewhere that will either slow down or speed up the decay, making it difficult for them to pinpoint the time of death.
Drug them with a large amount of Gamma Hydroxybutyrate or stab them with a sharpened knife made of ice. But in the process, make sure you don't leave DNA like hair, skin cells, finger prints, nose prints shoe prints or ear prints or fibres from your clothes, dirt/mud from the bottom of your shoes etc.