Hard mode
Unlock all 36 licenses in arcade mode under the normal difficulty setting, then press Circle while at the level selection screen.

Bonus levels
Win a gold medal in all X Games competitions to access the Deep Freeze Mountain, Grind Or Die, Halfpipe Winter Style, and four more Slopestyle Events.

Night levels
Finish all four minute events in X Games mode (SnowborderX, Superpipe, Big Air, and Slope Style), to access Big Air and Super Pipe at night.

Tricks

Back Flip: Press Down.
BS Rodeo: Hold L1 or L2 + Down/Left. Press Down/Right if goofy.
Corqscrew: Hold L1 or L2 + Down/Right. Press Down/Left if goofy.
Front Flip: Press Up.
Indy Grab: Press Circle.
McTwist: Press Down/Left.
Mute Grab: Press R1 then press Circle.
Method: Press R1 then press Triangle.
Rodeo: Press Down/Right.
Tail Grab: Press X + Square.

Hints & Tips

Keeping sponsors longer
Once you win a competition in the Rank B contests in snowboarder mode, a person will advance and say that they want to sponsor you. Accept, and swiftly save the game. If you lose the sponsor, quit, and reload from the last save right away. You will have the same sponsor you had prior to them dropping you.

Winning more contests
This trick works in the Triple Air event (Rank B). Do Misty Twists, and as you start to flatten out (look more vertical), start spinning into 1440s, 1620s 1260s and so on. After the three air jumps, you should be in a position to at least score more than the high placer.

Gaining more sponsors
First get a sponsor. As you win more contests, more sponsors will hook you up. If you refuse an offer, the person with the offer will ask if you will repeat what you said. If you accept it, they will be happy if not, they will look at you and say "we thought it was a good offer".