${AMP} for phase 1, and $[AMP] for phase 2.
Typically the curly brackets are used; but sometimes the square brackets are what works, depending on what part of the page is being processed first. When you don't have the Two-Phase checkbox selected, just use the curly brackets.
The wiki as Sharma linked is an excellent resource for basic information. I would use their search, it's pretty good.
Side note on the ${AMP} is that is does insert two ampersands, so if you need to make a single ampersand (as SN strips them out); You'll either need to split the variable, or create your own custom jvar for the basic HTML characters.
