That is a great question, and something that could lead to injury if not done properly. When we are talking about posting up, we must first understand why we have to post up in the golf swing.
First, we must post up in order to have a more stable clubface at impact (this leads to more centered strikes on the face), we also need to post up in order to have an effective workhorse. The workhorse in our golf swing is the lower body, and how it engages the ground in order to not only initiate the swing, but to also deliver a lagging clubhead to the golf ball.
In order to avoid injury, a slightly flared front foot towards the target will help alleviate any torque that puts your knee under stress. Above all, having your weight shift into the heel of the front leg, (the leg that is posting up) will keep you from over torquing that front knee. Injuries in posting up come from having your weight slammed into the outside of the front foot or even having that weight get out onto your toes. If weight gets shifted into the lead heel, your knee does not get stress.