Pause the game to give a skill to a lemming,
particularly if he's near a crowd. You can tell
which lemming will get the skill because he'll be
highlighted when you hold the cursor over him.
The game will immediately unpause once you click him.
Use blockers to corral most of your lemmings when you
have to watch a particular activity closely, such as
building stairs or timing Bombers.
Some levels appear impossible until you figure out how
the triggers work. Do a test-run over all the laser and
levers before you give up.