This is a little trick I learned when catching the lake trio, bird trio, and Cresselia in Platinum.
Go to a city without any gates. Get on you're bike and ride back and fourth between routes. I'm going to use Camphrier Town as an example (though I suggest you use that as a good start). I would start there then ride my bike back and fourth through the town and the routes surrounding it. You can stop when you realize Zapdos is on the same route as you then you can just ride in the grass for it. Then rinse and repeat!
Good luck catching Zapdos and may the odds be ever in your favor.