I find that it's best to leave getting ready for school until the last minute (for the girls) but be prepared long before (for me).
So: We have a 'destination station'. Each girl has a coat peg. They have a slot for their school bags, a slot for gloves/hat/scarf/etc. and a slot for shoes.
The key with this bit is to make them put everything in the right place the night before as they come in from school/go to bed.
I try and make sure I write in DD1's home-school book the night before, so I just have to add any crucial sentence in the morning. Likewise any envelopes with forms/money all sorted the night before.
They can do whatever they like until I say 'get ready for school.' Then, they have to get uniform on, shoes on, and sit at the dining room table. DD1 does much better if she can have scones for breakfast, and needs longer for cereals.
I literally wait until 30 minutes before her bus comes:
7.50 -uniform on.
8.00 -breakfast.
8.10 -hair.
8.15 -coat, check face, bag.
8.20 - get on bus.
Then I have another 10-15 minutes to get the other girls finished off.
8.35 - out the door.
I set myself a different time to the girls. I start chanting about being late at 8.30 so that they hurry. I know that in reality as long as we leave by 8.40 we'll be ok.
Also, I use a mixture of incentive - 1 dolly mixture for getting x done and fear (bad mummy) - I am starting the engine in x minutes.