Generally, you will find adverts for this type of service on the Internet, in technology buildings, in the phone book, newspapers, and as many technology related areas as possible.
When data cabling contractors advertise their companies services in the areas that people are most likely to need that service, the amount of customers will increase. This is called niche marketing and as a data cabling contractor it is going to be crucial to your success.
If you market your business in the wrong areas then this is quickly going to become detrimental to the company and your client base will steadily decline.
You are advertising a technology related service so obviously areas where technology oriented people are commuting are going to be your prime advertising areas.
Get out a sheet of paper and write down everywhere you can think of that tech savvy individuals and businesses would be at. Whether this is your local electronics shop or company offices, wherever it may be you should write it down.
You will need to evaluate everywhere you have brainstormed and pinpoint these areas down to exact locations. From there, you should have built a strong list of your best places to advertise.