I always forget how to counter a crossup…

How to counter a cross-up or how to block one? Some moves like teleports, Abel’s roll, and some special moves (like Dee Jay’s EX Jacknife Maximum, Hakan’s Ultra 2, or Blanka’s EX Up-Ball) can be good cross-up defense if timed properly. To block a cross-up, simply stand and block in the opposite direction. Be careful with ambiguous cross ups, though. Those can be hard to see.

Which game? Which characters?

block in the opposite direction from where they’re crossing^.

Like another user said, some characters have get out of jail free cards (i.e. shoryukens, teleports, etc.) The general way to do it is this: Say your opponent is on the right of the screen. He tries to jump over you and cross you up. You would block right yourself. You would block in the direction he was originally jumping from since the crossup would hit you on the opposite-side. Make sure you are stand-blocking though and not crouch-blocking. That might be your problem.

Which game because reacting to cross ups in certain games are different from lets say SFIV.