$5 for helping with bags, assisting passengers or facilitating a quick turn
$10 if "exceptional" service is provided
$20 for a lav service

I've also been known to ask if the desk gals want a Starbucks or something in lieu of a cash tip; sometimes their service contribution is missed by pilots (except when Captain Studley hits on them for 20 minutes) and a simple acknowledgement can go a long way to great service if you are a frequent visitor.