Whenever you sync your iPhone with iTunes. iTunes delets everything off the phone and replaces it all with whatever is in iTunes.
the fix is simple.
Before you plug in device, open itunes
in the top left corner click: Edit
Then click "Preferences"
Then click "Devices" (the iPhones icon)
Then near the middel of that page click on the box beside "Prevent iphones, iPods, and iPads from syncing automatically" so there is a checkmark in that box.
Now your phone will not sync unless you want it to.
Now plug in your iPhone.
On the left side where your iPhone appears after it's plugged in, right click on it.
then click "Transfer Purchases"
Wait til that is done. That is how your transfer your purchased items from iPhone to iTunes. Syncing replaces everything on the phone with whatever is in your iTunes library.
The "right click on device and transfer purchases" is really all ya had to do to get your purchaes from iPhone to iTunes, but the other settings I told you to apply prevent anything from being deleted accidentally.
Also........What i suggest every iTunes/iPhone user to do is to click in there device on the left side of iTunes.
In the large mailn window, click "Summarry" on top. (usially don't have to as that is where it already is when you click on your iPhone)
Put a check mark in the box that says "Manually Manage Music". (Also applies to apps)
This way you can drag and drop individual apps or songs (or however many items you selected) to your device without having to sync a whole section of Music or apps.
But in a nutshell, All ya have to do is transfer purchases. i suggest you do this every time you buy something on your phone so it is always backed up on iTunes.
There is no need to sync your iPhone to iTunes unless you want your iPhone's content to be replaced with whatever is in iTunes.
just transfer purchases..............And you task is done