I think showing that you are serious about getting a dog and are prepared to share responsibility in caring for it.

My dad was opposed to many pets that we had... mainly because he was right -- we'd get it home, love and help take care of it for the first few weeks, but after the honeymoon phase was over everything ultimately laid on HIS shoulders to care for it. And he *always* got stuck cleaning up in the yard after it.

Ultimately though, your dad's house, your dad's rules.

This may sound kind of lame, but put together a presentation.... a breakdown of finances, things you will do to help raise money to care for the dog, cost for training and how you will help out with it, etc; get together with your sisters and put together a schedule of responsibilities, ie, you will clean the yard on Wednesdays, your sisters clean the yard on Saturdays, a schedule on who will be walking the dog, who will be feeding the dog, etc.

Have a fundraiser or offer to do extra chores to help raise money to put into a vet fund to cover the cost of puppy vaccines, training, etc.

If you go to your dad with a game plan that already covers everything, he may take you a little more seriously and may consider it.

If he is still insistent against getting a dog, the most important thing is DO NOT BEG. Be mature, say, ok dad, is there anything we can do to sway your opinion? And if he says no -- DROP IT and revisit it at a later time.

But the absolute most important thing is do not go behind his back (my mom would ALWAYS do this to my dad, which created resentment of my dad toward the pet, not to mention getting into arguments about it) and do NOT beg or whine if he says 'no'. Respect his decision and be mature and he will be more likely to take you more seriously next time you present the idea.