One last tip that bit us hard: beware of universal apps (apps designed for both iPhone and iPad).
Once you've published your app as universal, there is no way back. You can't decide in a subsequent update to revert to an iPhone-only or iPad-only app - Apple won't let you.
So if you start with an iPhone app, and then put together an iPad version of the UI just to try out and publish it as a universal app, you'll be stuck with having to provide an iPad version of your app for the rest of times. Even if you realize that the iPad version isn't worth it for your app and that nobody uses it, you won't be able to discontinue it.
So think very carefully before flicking that switch.
You also won't be able to charge different prices for the iPad and iPhone versions either. Although you probably could make separate in-app purchases that get offered based on device type.
Once you've published your app as universal, there is no way back. You can't decide in a subsequent update to revert to an iPhone-only or iPad-only app - Apple won't let you.
So if you start with an iPhone app, and then put together an iPad version of the UI just to try out and publish it as a universal app, you'll be stuck with having to provide an iPad version of your app for the rest of times. Even if you realize that the iPad version isn't worth it for your app and that nobody uses it, you won't be able to discontinue it.
So think very carefully before flicking that switch.