Capstar has been a mircale pill for us. 3 hours visiting family and Zoe got INFESTED with fleas. She'd had Frontline applied 2.5 weeks prior but it didn't make a difference. We went to a local vet and got a pill .... it made me so happy to see dead fleas!

Other than regularly using something (Frontline, Advantage, etc) capstar is a good immediate killer of fleas (but not eggs).
Also keep in mind that Comfortis (sp?)
is a prescription! For instance Zoe is 5.5 pounds but has had a weak stomach in the past as a puppy so she can't have any, per the vet! So be careful just giving to it to another dog, they may not be able to handle it!
Good luck everyone!