Don't own a bike.
But for real, I had my bike stolen this last week and after watching countless videos and reading a plethora of blogs and bike lock reviews, it seems that the only sure-fire way of keeping a bike safe is to A) don't own one B) store it inside.
Of course, most people who own bikes don't want to roll them inside every time they use them. Having a tiny studio apartment also doesn't help. So, I'm like the other fools, locking my bike up outside.
I will admit fault for not realizing how easy it is to cut through locks though. Wire cutters or bolt cutters (depending on how thick your cheap lock is) and you're done. Bike thief is riding off with your bike and you can pretty much kiss it goodbye.
Sure you can file a police report and see if your insurance covers theft, but nothing will fill the void left that your rusty mountain bike from Salvation Army filled. Except maybe a new, Walmart bike that cost less than the price of the two new locks you bought. Bike Thief is gonna have to work way harder to steal this new, not-worth-their-time bike.
The reality is, if you actually want to make it difficult for thieves, you're gonna have to spend a lot on some decent bike locks. Get at least a 16mm thick U-Bar with good reviews. Probably also get a chain lock or a cable to protect your front tire. Use the U-lock to lock your frame and back tire to an immovable object and make sure not to leave much room inside the U-lock. There's this great trick where shadier characters jam a modified car jack into the U part of the lock and crank it to bust the lock open. Don't leave room for that car jack to fit.
Anyway, I found this website an awesome resource during this whole process.
Long story short: Don't use a $15 combination bike lock and expect it to deter anyone even slightly determined to get your bike.