No cable locks. Bad, and you get what you deserve.

Something that's a pain in the ass to cut. Nothing is impossible to cut, but a decent u-lock, good chain, good folder, is the way to go. if you are regularly locking up in a specific place, you might be able to leave a heavier lock (chain) there and not have to haul it around.

Something noisy. A brake disc alarm, or two of them, that go off with movement. Getting attention is good.

If it's an e-bike, take the battery with you.

My goal is to make stealing my bike more of a pain than moving on to the next bike, probably secured with a cable lock from Walmart. It might take five minutes to steal my bike and 20 seconds to steal the next guy's, about all you can really do.

Finally, have insurance. This world is full of fucking assholes.

I run pitlock skewers on my bike with an expensive dynamo hub. Expensive but worth in IMHO. Don't forget the key or you won't be able to replace a tube! I keep one key on my keychain and one tucked in my multi tool JIC. I have also heard of people in my area super glueing a ball bearing in any recessed bolts to prevent an allen key from fitting but can be popped out if you know it's there. Think places like the headset and seat clamps.

https://www.sheldonbrown.com/lock-strategy.html

College campuses are like buffets for bike thieves. Outside of that, most bike theft is a crime of opportunity, and just being locked better than the bikes next to it is enough. But on campus (and other places with heavy use, like outside museums), lock both wheels to the frame and the frame to a rack, and lock the seat, too.

Never lock your bike to street signs if you can help it. They're easy to disassemble. Use a proper rack.

Swap out any "quick release" mechanisms for the kind that use tools.

Get a strong chain or a beefy cable. Fuhgeddaboutit locks are very reliable. Make sure to wrap the chain/cable so that there is as little free space to get a set of chompers around it as possible. Make sure to include at least front wheel, if not both wheels, in the wrap. Try to make sure yours is either better secured or less valuable, or both, than your neighbor's. Install a discreet gps locator. Don't leave valuables attached to the frame while it is not directly in your custody.

• Stay away from locks with barrel keys.
• Choose a locking device that isn't easily cut/broken/defeated.
• Have a way to pass your lock through your front wheel AND frame as well as what you are locking it to.
• Don't lock up to signs.
• Make sure what you are locking to can't be easily moved (preferably bolted or sunk into concrete slabs)
• Don't block pathways

Get yourself a chain lock. Put it around your wheel/frame and the bike parking frame thingy or a lamppost. Should keep your bike safe in most cases.