The best way to protect your bike, in the most cost effective way would be to purchase a U-lock. It is a medium duty lock for a relatively good price.
Prices range, but the average is about $29.99 which when you take this cost against that of losing your $300 bike, it would seem worth it.
You can buy these kinds of locks from most bike stores, or hardware stores.
If you are more the internet person I would check out
http://www.kryptonitelock.com/
They make a pretty decent lock.
Also if you want more information on the durability of several types of locks check out this page:
http://www.slate.com/id/2140083
The author tests the abilities of multiple locks.
I personally have heard testimony to the U-Lock.
It seems to be legitimate.
Keep in mind that there is no such thing as a theft-proof bike lock
Only more protective.