As you approach the location of a boss battle, there are almost always two boxes nearby. One box will contain health, and the other a battery recharger. If you enter the nearby save room (or simply leave the area) and return to the location of the boxes, both will return, full of items for you to grab once more.
Here are some basic Wii remote controls for your motorcycle that you may not know about. For instance, if you're riding full speed on your motorcycle, try tilting the Wii remote upwards. This will allow you to jump cars and other obstacles.
Also, if you press the brakes suddenly and then tilt your Wii remote to the left or right as you do, your motorcycle will quickly tilt 90 degrees in the way you were pointing. The use for this is obvious -- quick turns!
To see the game's real, "true" ending, you must collect all of the game's Beam Katanas.
To unlock a harder difficulty (called "Bitter"), beat the game once through on a lesser difficulty level.