I use a thick bike chain lock. Kryptonite 1016 Evolution specifically. It's also fairly long at 5.25ft/1.6m, which allows me to chain it around small trees. After reading some other replies I think I'll be adding a u-lock too. I also always lock it up in a visible area.
It's about sufficient deterrence, I think. Which is situational. The longest I ever need to leave my bike unattended is an hour. But usually less. I feel my security is sufficient as it's difficult enough to defeat that a potential thief couldn't do so without drawing unwanted attention. But I wouldn't be comfortable with that as a solution for say an 8 hour stretch of time.