Every chance you get, do not kill the human before you extract their brain stem. Extract their brain stem first as it will make the DNA worth more (25 instead of 10).The easiest humans to do this to are the scientists in the level with glowing cows. For some reason, if the person that is important to you dies (the person you need to abduct), extract their brain stem before you return to the Mothership -- the DNA is usually worth more.