When on Forge place an active Gravity Lift somewhere where the Plasma Grenade hits a wall or ceiling. Aim the grenade at the base's middle section, or at the wall so it falls down. It should stay there unless it's touching a weird section of the surface.
This trick is easily done on the Halo 3 multi-player Isolation where the side areas can be used.
Note: The grenades can stick to each other, causing them to explode. So stay back when doing this. You can also run into the plasma grenade, fun but suicidal.