For a little bit of drag without needing to adjust
frequently, a light spring inside the collar would
add just a little pressure, and would take up
the slight wear.

An alternative is to put a little toe-in, which will
force the wheels against the collars. The pressure
decreases as the plane slows down so it doesn't
cause nose overs.

An added benefit is that if the plane wants to wander
during takeoff or landing, the wheel that is pushing
hardest gets more drag and pulls the plane back
into a straighter path.

Jenny