Only chemicals will work unfortunately. I use Revolution, but Frontline plus is cheaper and is for fleas only. I get mine from eBay and it's heaps cheaper. A 6 month packet for a 10-20 kg dog is usually about $120 in the vets, but it's around $80 from eBay.

Also it says one application every month, but sometimes I've done it every 6-8 weeks and my dog hasn't even started to scratch. So you can stretch it out longer.