1-up Trick:

Continuiosly stomp on various enemies. You'll rack up points, eventually it'll be 1-ups you're racking up. This trick also works in SMB & SMW!

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The White Block Trick:

In various stages that have those white platforms, jump on one, duck & stay down for several seconds. You'll be able to travel through the background unharmed in most cases. This'll last a few seconds.