Lots of Ammo
When in a large gunfight against multiple enemies, there's a little trick you can employ to get nearly unlimited ammo. Instead of going out and taking on all the enemies, crouch near an ammo case and fire from there. When you get low on ammo, just press the "+" button on the Wii remote to fill back up. This has the added advantage of forcing enemies to come to you, giving you the upper hand. Obviously, the ammo crates are not unlimited, but they'll usually last you far longer than you need. This trick is very useful in the final boss battle against Greene.
Easy Kills
There are several sections in the game where you'll be forced to rely on stealth. In these sections, you can save your ammo and get easier kills by sneaking up on enemies from behind and smacking them in the back of the head. Not only is this an instant kill, but it won't alert any other guards. On Level 1, you have the option of killing a guard without actually shooting him; just shoot the glass panel he's standing on, and he'll fall through to his death.
Infinite Ammo
To unlock infinite ammunition throughout the game, defeat the entire single-player story mode on "007 Agent" difficulty. Once you finish, you'll be able to turn the unlimited ammunition on or off via the "Options" menu.