just dump a little nitro fuel on them. they will swell up and then come right back to original size. the fuel makes the foam expand cleaning the dirt out and then tightens it back up when it contracts.
i use belray air filter oil, it protects 50,000.00 mx bikes, so i am sure it will do the job for my 500.00 engine.
i have 2 filters one for quals one for mains. and i cleaned them several times like 20-30 times before pitching them.