5 Feet of Fury

Always place the tip in the server’s or driver’s hand

Exactly:

If you want a better chance of your server keeping the tip, always tip in cash. If you see your tip as a very specific thank-you—a host managed to sneak in your party of 10 last-minute, a bartender spent all night mixing off-menu drinks for you, a server pulled out all the stops for your date’s birthday—that’s best handled with a bill or few slipped into a handshake. (Though even that should be on top of a standard, on-table tip. Or you’re stiffing those food runners again.)