The Income approach as Jim describes is correct for most commercial property. If yours has been vacant for quite awhile, your income statement will be adversly impacted hence lowering the value obtained from that standpoint.
If you have multi-family properties (or a couple other specific types), a Sales Comparison approach could also be used. For example, an apartment building sold for $50,000 per unit, or $X per square foot. This approach is most often used for residential property.
If your property is something very unique for the area and/or there's an option to build instead of buy, the Cost approach could be used. For example, if it costs $100 per square foot to build a building like yours in your area, multiply that by the square footage of your building and add the site costs (raw land plus improvement costs) to arrive at a value.
All of these approachs are what an appraisers and brokers can use to value a property. If you're still unsure about what the value might be, running these numbers, have it appraised.
Mar 9, 2011