Making iPhone/iPad the master when syncing with iTunes

I am syncing my iPhone with my Mac using iTunes. How do I make the iPhone have priority over the Mac? I mean, I want to copy what is on the iPhone to iTunes, deleting from iTunes what is not anymore on the iPhone.

I hate when I clean my phone, do a sync, just to have iTunes copy everything back, because iTunes database regarding to that phone is not the same.

iTunes gives what is on iTunes a priority and I want the reverse.

thanks.