The customary tip for taxi drivers is 15%–20%, with a minimum of $2. Bellhops are usually given $1–$2 per bag. Hotel maids should be tipped $2 per day of your stay. A doorman who hails a cab can be tipped $1–$2. You should also tip your hotel concierge for services rendered; the size of the tip depends on the difficulty of your request, as well as the quality of the concierge's work. For an ordinary dinner reservation or tour arrangements, $3–$5 should do; if the concierge scores seats at a popular restaurant or show, or performs unusual services (getting your laptop repaired, finding a good pet-sitter, etc.), $10 or more is appropriate.
Waiters should be tipped 15%–20%, though at higher-end restaurants a solid 20% is closer to the norm. Many restaurants add a gratuity to the bill for parties of six or more. Ask what the percentage is if the menu or bill doesn't state it. Tip $1 per drink you order at the bar, though if at an upscale establishment, those $15 martinis warrant at least a $2 tip.