I like to keep tension on the biceps when "down" as opposed to a complete lock-out -- if I didn't get this just right while in the Marine Corps, then the pull-up wasn't counted (during a test) -- occassionally, but rarely I had one or two not count before I reached my 20th good one (once, when a lot of folks were doing pull-ups on a large frame that was comming loose I ended-up doing 42 to get my 20 good ones). My arms flare out more than most guys (a condition common with most women... does that mean I'm a girly-man?... I digress), perhaps this is why my elbows give me problems when I try locking-out.
My "trick" to avoid initial lock-out was to jump up high & grab the bar with semi-bent arms & then come down a bit with control until it looked like I was locked out (but I really had my biceps tensed at near-lockout).