Pre-tickle attack banter is a fun way to elicit those sweet anticipation giggles. Some lines that I regularly use to signal an impending tickle attack:
When laughed at, or being teased: "Oh, so you like to laugh, eh?" while wiggling my fingers. The transition from joke-induced laughter, to ticklish-anticipation giggles is hilarious.
When being playfully antagonized: "Somebody wants to die laughing." Or, in a cool and even tone (I channel Agent Smith, for this one), I'll go "If you're ticklish, I advise against repeating that action..."
When in a particularly impish mood, I'll pretend to offer help by going "You have something on your shirt, let me get that..." and then while tickling them, act innocently helpful, and pretend to be annoyed at their ticklishness.
As you can tell, I'm about as subtle Cookie Monster 😛