SLAYER:

        At the end of level 1 make sure you only have one life left and
        knock out all of the guns and green monsters, then crash into the
        machine the monster was in.  You'll start level 2 with infinite
        lives.

        Confront the end of level gaurdians with one life intact.  Press
        'D' then crash into it's dead body.  You'll now have infinite lives.