How to upload data to my iphone from my computer without wiping the phone clean?

Some time ago, I purchased a macbook air (beefed up ipad), and have downloaded some movies from itunes to it. I hooked up my iphone, and synced pics, calendar, phone book, notes, etc, and I want to upload a movie from my itunes library to my phone.
Everything is 'ghosted out', and when I go to click on 'sync all movies', It tells me everything on my phone will be deleted and the library on my computer will be synced to the phone.
I don't want to lose 28 gigs of movies and music on my phone.
How can I upload movies from my mac to my iphone without losing all my data?

Make a bakup of your precious treasures:
http://support.apple.com/...kb/PH12125 (Backup iOS device)

Then manually manage syncing…
http://support.apple.com/.../kb/HT1535 (Manually manage sync)

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