There are many technical duties and tricks to being an efficient 2nd AC, e.g. proper distance of the slate from the lens, clean and thorough camera reports/timecards, doing everything to insure the 1st AC doesn't have to leave the camera, being a ninja with actor/focus marks, a good courtesy tab on the tape rolls, etc. Most of this comes from just spending time on set.
But everyone previous has mentioned some important qualities that are less technical and more personable. Being a trustworthy, loyal, mature, sociable person goes a long way on the set.
Thinking ahead is invaluable. Being there before your colleague has a chance to make the request saves time and effort, and inherently helps you become a well oiled machine.
cheers~