On stage four, when Sheldon head butts the enemy, he earns Garfield one invincibility. Just starting summoning Sheldon and let him hit the enemy. DO NOT WIN THE LEVEL BECAUSE THIS CHEAT ONLY WORKS ON STAGE FOUR!
The popcorn machine provides the currency you use to summon in Garfield's friends. Through out the game it's important to maintain a decent level of these resources for when you really need them.
There's no point spending all that precious popcorn early on in a level, so wait until the action heats up.
You'll want to have a lot of popcorn in the bank so you can bring Orson into play as soon as possible.