If you don't mind getting a little cheaty you can use AMIDST (Advanced Minecraft Interface and Data/Structure Tracking.) Otherwise, there is no "legit" way besides "look around some more." However you can be a bit smarter about your looking -- nether fortresses tend to generate in "strips" along the north-south axis (z-axis) so you should explore either east-west or diagonally, lest you end up stuck between two of these strips and never cross paths with one at all if you just go north-south.
EDIT: If you're using AMIDST, note that the nether fortresses are displayed via their nether-coordinates, but on the overworld map. This could be a bit misleading, because of the 1:8 difference in scale between the two worlds. If you go to that location in the overworld and build a portal, you won't be near the fortress. You need to go to the nether, and then travel to the fortress, or else just find the coordinates for the fortress, multiply by 8, and build your portal there in the overworld.
Example: Your base is at coordinates (200, 400). AMIDST says there's a nether fortress at (400, 400). What this means is it's at that location in the nether. If you go 200 blocks east from your base and build a portal, you won't be anywhere near the fortress. Instead, you'll be at coordinates (50, 50).
To find the fortress, you can either build a portal at your base, and enter the nether at (25, 50) then travel to (400, 400), or you can travel in the overworld to coordinates (3200, 3200) and build the portal there, which will drop you out at (400, 400) in the nether.
Village Mechanics: A not-so-brief guide - Update for 1.4! Added subsection on iron farming to the iron golems section. Only ten villagers required to spawn golems (down from sixteen)! Put to rest the "facing doors cancel each other" myth!
AMIDST is a good program, another way is to just go creative and fly around. Note that nether fortresses tend to spawn along the Z axis so fly along the X axis until you find one and then all other forts will be along the Z axis of that specific X coordinate. Then using this rule of thumb, you can find the Fort closest to your portal.
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