A creature can end the prone condition on itself by standing up.
Action: Move action.
Unoccupied Space: If the creature’s space is not occupied by another creature, the creature stands up where it is.
Occupied Space: If the creature’s space is occupied by another creature, the creature can stand up and shift 1 square to an unoccupied space. It can’t stand up if no unoccupied space is adjacent to it.
Even a prone creature that is limbless or amorphous can take this action, which represents the creature righting itself or regaining its balance rather than literally standing.
Published in Player's Handbook, page(s) 292, Rules Compendium, page(s) 250.