If you have your printer calibrated properly, and slicer settings set up right, your printer should be able to print bridges (unsupported filament) that do not sag. You may want to download a bridging calibration piece to try out your settings and adjust them as needed. Some people also find a fan blowing on the plastic as it's extruded helps. Slic3r has a cooling option that lets you tell it whether to activate a fan for bridging or not.