Get a can of your favorite scent of Lysol, spray some in the cabin air intake at the rear of the hood while the engine is on and A/C is running with the blower on high. I only use a small amount, I've heard of people using a whole can or more. The Lysol will kill most of the boogers that create the smell, lasts for one to three months. I like the linen scent, but you pick your own. The scent will stay for a few days, longer if you use a lot.
Somebody (Geno's?) sells an air filter that you mount outside on top of the air intake, but I don't think that will help given that the smells usually come from inside. It might help in the future if it keeps dust, pollen and other nasties out of the system.