we use the Revolution topical once a month. It has the flea prevention and the heart worm meds in one dosage. It is dosed according to weight. 1-5 lbs., 5-10, etc. I have used it for years and my girls have never had a problem and no fleas or ticks ever.
1800 pet meds carries it and there is a discount if you click through this link:
1800PetMeds.com - America's Pet Medication and Pet Health Resource