3. GET SOME ALLIES

Power in numbers is pretty good advice when dealing with security. This won't require forming a militia (sorry to get your hopes up) but rather becoming friendly with two groups of people: neighbors and security guards.

Neighbors
Security guards

Neighbors

You know the woman that has sixteen cats, smells like Lysol, and only eats Good 'N' Plentys? Well, besides being your neighbor and source of free entertainment, she can also help you guard your apartment from crime. Besides getting your mail and newspapers when you're on vacation, your neighbors can keep an eye out for suspicious activity around your apartment and alert the police if they see any. And as we already said, consider giving a trusted neighbor a key to your place so he/she can check in periodically when you're gone for a while.

Also, don't be a jerk -- you should reciprocate. If someone wants you to keep an eye on their apartment, just do it.

Finally, the more people you know in your building, the easier it will be to spot suspicious "visitors." So make an effort to meet people and learn a little about them. Not only will you make friends, but you'll also have formed your own little neighborhood watch.

Security guards

If you have a security guard in your apartment, you should take advantage of this to the fullest. Get to know him/her - if you do, it's much more likely that he/she will keep an eye on your place and monitor visitors to your apartment. It's also a good idea to advise the security guard to NEVER let anyone asking for you into the building without your permission. Finally, let the security guard know when you'll be on vacation. That way, if Shady Repairman (cue evil music) shows up, the guard will know it's a ruse to steal your precious ceramic broccoli collection.

If you think that your security guard spends more time scratching his/her butt than securing the building, don't be afraid to speak up to the landlord or the security company. It's your building after all, and you're paying for that security guard with your rent.

Finally, if you don't have a security guard, think about petitioning your neighbors to get one brought on. It may increase your rent, but it's definitely worthwhile. Security guards not only help keep your stuff safe when you're gone, but they also allow you to feel safe while walking into the building, and they might sign for any deliveries that you aren't home to get.