Any modern book on etiquette published in the States will cite either of these two historical figures.
For more modern situations, you may consult the Emily Post Institute, which will help clear up any confusion on how to best represent yourself in America.
Say something like: "May I have some change for the tip, please? " Tipping is also common in the service and hospitality industry, from valet to housekeepers at your hotel.
$2-$3 tips for valet each time you retrieve your car (none when you give it to the valet) are common.
$2 per day for the housekeeper at your hotel, left in the room at the end of your stay is common.