Respectfullness
Don't walk in the FBO acting like a wiseguy. That should go without saying. Walk in, dressed appropriate to the job, and leave an updated resume. I think you waited too long. I see nothing wrong with going over once a month or so.
You do have to be persistent. After a while, though, if you constantly get the cold shoulder, look elsewhere because it just may not happen at that company. I know. I tried for six years to get an interview with SkyWest, GP-Express, Horizon and at least one or two other commuters. Not to mention all the commuters who which I sent materials and updated for shorter periods. I sent in apps, resumes, updated resumes, new apps, had a former student walk in materials to SkyWest, and got zilch.
Just hang in. It also helps if you make friends or develop a relationship with someone at the FBO to vouch for your being a good guy, etc.
Best of luck with your job search.