If it was kids, you shouldn’t worry too much, as they typically are just one timers who stole some stuff for the thrill of being thieves and usually do not try to come back. If it was adults, they may have scoped your place out, knowing you keep the place unlocked, and they may even live close to you. If you aren’t anti-gun, get some training and get a gun. If a burgler thinks there is even the slightest possibility you or someone else may be home and armed, your risk of burglary or home invasion goes down significantly.
Make friends with some of your neighbors, and they can help watch your place.
A dog is a good idea if you can have one in your apartment. Most places you can’t, especially the kind that barks and would be good against an invader.
I’m in an apartment in a very good neighborhood and I still have a monitored alarm through Orca, because if I leave on vacation I don’t want to worry about not knowing if someone broke in. Sometimes these things can be more trouble than they are worth though, and it should never be your only plan.
Get good insurance. Get a safe that is fairly heavy and if you can mount it, do that. A burglar may be able to lift and run off with it, but they’d certainly look suspicious leaving your place with it.
But ultimately, don’t keep anything too valuable around. If you’re afraid of losing it, you should probably keep it in a secure vault.
Now, as for people breaking into my car, I’m a freaking expert on having it happen to me. Back when I lived in Lakewood, I had it broken into 5 times, and once was just to steal a smelly old blanket I had in the back-seat. I had to replace my passenger side window three times. Had someone use a coat hanger or slim jim once, and when one of the windows was busted out, had someone tear out the plastic and steal my air freshner. This particular car was finally finished off by a 16year old girl who decided to drive into it at 40mph while it was parked. I then got a used car with the money from the insurance, and someone the next week side swiped it while it was parked and ripped off the front end and crushed the fender.
I now have a garage.