Everyone works somewhere, and if you own commercial space people might as well work in your building and pay you for the privilege. If you have a vacancy, you may be tempted to dwell on the abundance of commercial space available, but that mentality won't help you rent out your space. Instead, focus on what you can do to make your rental attractive and how you can draw attention to it.
Improve the space. Commercial tenants rent to conduct business. If the commercial space you offer helps them to do their business, they'll be attracted to it over any other potential rentals. This might mean redoing a facade to attract customers, adding landscaping or a water feature or providing nice office furniture. If you don't have the means to improve the space or if it's not practical for the building, make sure the rent is set attractively.
Advertise in the newspaper. List the features, provide an address and make sure to include a way to contact you or your agent.
Advertise online. Put an ad on craigslist or on a local classified site. If your building is suited to a particular trade, it may benefit you to advertise on websites that cater to that trade. Include pictures where possible.
Place a sign on the premises. Make sure it's large enough to attract attention and that it looks professional.
Call local businesses. Let them know that you have commercial space available that you think would suit them. There are any number of reasons they might be interested in your property: they might need more or less space, their lease may be ending soon or they might want a new location. You never know until you call.
Call out-of-town businesses. Explain the benefits of being located in your city and ask if they'd be interested in moving or expanding.
Advertise in trade magazines. If your space is perfectly suited for a particular business, place an ad in a magazine pertaining to that business.
Actively pursue tenants; don't just stick up a sign and wait for them to call you. If you're too busy to call businesses, hire someone to do it.