Most marketers/advertising agents/sales people will tell you you advertising budget should be 4% of your gross sales each year. It is an investment not an expense, unless you are not picking up new clients. Each location is different, what works well in one area may not be good in your area, so be careful. Direct mail is the most cost effective, becuase you can narrow down the criteria for the type of client you want to reach. But just becuase the direct mail is reaching the prospective client doesn't mean they are taking the time to read it or look at it. So what ever adterising you use has to grab thier attention immediately, this may involve being in the top 5 or 10 positions in the handouts, which might cost more money.
I have always relied on growing my business via referrals from existing clients. It is a slow and sometimes tedious process but it puts you in contact with the types of people you are already doing buisness with. Let your existing clients know you are wanting to pick up new clients. Your clients want you to succeed and grow. It would also be wise to join a property management group in your area. This puts you in front of area property managers, front and center, and gives you the opportunity to market your business to the decision makers in the commercial property arena. You may not get the premier property right off the bat, but it gets you in the door and all it will cost you is a membership fee and some time after work socializing.