A great deal of it is also how you dress and present yourself. If you walk in and you're (1) dressed down compared to the average person there and (2) either look to humble or act too cocky...well, that $20 isn't going to get you very far. My advice:
1. Stick with the $20, discreetly passed during a handshake.
2. Dress the part...plan on dressing better than you initially think you have to for that place.
3. Be nice, but somewhat aloof. Don't be a dick, but subtly act as though they're lucky to have you there.
Also keep in mind that at a "hot" place, if you decide to just drop in (no res) at a peak time...they actually may not have a table. It does happen. IME it's always better to save the $20 for a table "upgrade" (to ensure a good one when you have a res) or for the doorman at at club, where two more can always fit.