In my own words... (lol)
France:
Napoleon needs to be served first... even though he stops you from being able to complete other customer's orders, you can still give them cookies and play the tv for them to increase their hearts.
If you get a lot of Naploeons in your store, serve them as quickly as possible, but don't pick up their money until all your other customers have been taken care of. Leaving the money on the counter will prevent new customers from coming in.
Use the multi colored button on the frosters to remove icing from a cake if you've made a mistake. This avoids losing money by throwing out a cake, but you can only use that button once per level.
Customers with BLUE thought bubbles (where their order appears) must be served first! If you let them stay in your store too long, they will make the level too difficult.
Don't forget the basic tips like:
Right click a cake while you have another in your hand, and they will trade places.
Chain events three in a row to fill up the Sugar Rush bar. Sugar Rush is great when you have a lot of customers with complicated orders, as everything is made as soon as you order it.
People with double orders gain hearts after you give them their first item. Use this to keep them happy while you serve others with only one order.
Customers that order a cake and a drink gain hearts after you give them one of their orders, use that to make them wait longer while you serve others.
Use your TV. Buy one if you haven't yet.
Taking phone orders can give you extra money you may need. Remember that NOT answering the phone does not penalize you, so don't be afraid not to answer it if you're already too frazzled.
Play the mini games if you haven't done so... you earn extra money for upgrades through them. Don't rush through the minigames... the better you do, the more money you get... restart, or redo it before you finish because you only get one chance at this easy money.