I keep 2, sometimes 3 bikes at work in the bike shed as the garage is full
Ideally, avoid riding anything worth having if you will be leaving it out in the wild, unattended, to be actioned by urban predators? Make them look for something else which looks to be worth more. I understand the sentiment that "Well, I paid for a nice bike, why should I let other people decide what I ride" but practically, the world is full of desperate people. Like anything, if it stands out, it will be noticed.
I read up on locks a lot last year. Avoid any
of those cables/chains with those circular key holes, as you can apparently ram the end of a biro in them and pop them open.
Next step up is a D-lock. Should deter the casual thief who's carrying cable cutters. More serious thieves will either have a piece of scaffold or will steal something similar nearby and force this into the D, and then twist, to lever or break the lock open. Branches or small trees, even.
But these are probably the best bet for carrying if you are locking your bike up in different places, where you can't leave the lock in-situ. Harder crims will use bottle-jacks to force the ends apart, if the space inside the D permits this - hence, the more closed the D, the better.
Next step up is a thick chain. Not really portable but can be left in your regular spot. Think about the hole in the middle of each link - can they get a thick bar in there to twist/lever the link apart? Go for something that doesn't make this easy - small holes where a thick bar won't go.
You'll need bolt croppers to do these, so will usually be organised/pro crims who'll have these, in a van. They'll probably not bother with cheap bikes as they can take posh ones and/or motorbikes which have more resale value.
You also need to think about the padlock - when locked, you want very little space around the clasped link, so they can't get anything in there to lever it. I asked a motorcyclist here and he told me about these folk below, who I must stress, I have no business or personal involvement with:
I ended up getting a 2-metre Immobiliser4 job, with the Squire padlock. It is very, very heavy. It lives at work. It cost about £200, but I can lock 3 bikes together with this - so less than £70 a bike, you are getting something that people trust to guard £15K's worth of Ducati.
Their site is interesting enough, just to see how little resistance the "Big Name" brands offer to bolt-croppers etc. I use my D locks now for securing the forks to the frame etc. Our site also has 24-7 security guards and cameras but a bloke did steal a bike a few years back, rode up on a piece of sh*t, pliers through someone's £5 cable, and off he went. 10 seconds the whole lot, gone before anything could be done.
Nowt gone since