my rule of thumb is.......if im riding something i care about or if i care about getting home.......lock it up with a ULOCK and never leave it from my line of sight or ....bring it inside with me.....

or you could try this:

http://www.dealextreme.com/details.dx/sku.27127

i just got mine in the mail. IT IS LOUD....although very sensitive. IF someone were to lock up next to your bike and even touch your bike....this thing will go off. Its cheap and makes alot of noise. This should keep people away Heck it is only 7 bucks.

Goofy....yes. Dorky....probably. Kinda cool....it makes the chip chrip noise They have one that has a remote as well just like for your car.