This is drastisc i know, but it works: lots of electrical tape spray paint, coffee grounds, sawdust and more spray paint. Be sure to tape/paint over all brand names and emblems so the bike looses its identity. Put random pieces of ducttape on a few cables and gear changer. Never wash it and always leave it a bit dusty. Basically make it look like garbage and not worth the effort. It should look unsellable. I did this after having 3 bikes stolen within a year. Now i can hopefully have it a bit longer. Protip: use foil paint /plastic dip so its technically reversible.

My bike is customized to the point that if you want to use / sell it you're going to have to spend a quite while removing duct tape and glue residue from it. I almost never leave it out of my sight except in my garage but when I do I lock it with 2 locks, U-lock and a chain-lock and I take the battery with me.

I have a very long electric lock (no key no number rotator) that I bind into both wheels, the frame and around the object I want my bike to stay at. Most thieves give up when they see such a lock and don't bother. They would need very good bolt cutters and walking up to a bike with these is usually very suspicious.

I have one of those $15 motion-activated alarms from Amazon (with a key-fob) buried under the seat of mine. That way, it will at least make some noise while it's being stolen. Obviously, use a lock (first and foremost).