Successfully complete all training modes with at least a Bronze rank to unlock "Bonus Scheme 1" in multi-player mode.
Computer worms never shoot you
To avoid having a worm shot by the enemy (especially useful in deathmatch and mission play), simply have your worm stand on their head. Unless that enemy worm's turn is next, they won't shoot you because they don't know when to sacrifice a worm to make a kill.
Home runs
Find an enemy worm standing fairly close to the edge of the level. Use the baseball bat to hit him into the water to get a "home run" complete with baseball bat cracking sound, baseball message (like "It's Outta Here", or "Bases Loaded"), and organ music.
Super Sheep Racing
If you have collected a crate at the very left of the level, and the next crate appears at the very right, deliberately crash or detonate your current sheep and set off a new one. This will save time.
Unlock all missions
To be able to play all the missions in single player mode, you must get a gold medal in the first training mode. After you have done this with a particular team, that team will then be able to play all the missions in mission mode.