I'm temporarily using my daughters 4s since mine went underwater. how do i back up my data on her phone without overwriting her backup file in icloud?

If you are syncing the phones to the same computer, turn off iCloud backup on her phone in Settings>iCloud, connect it to your computer, go to the Summary tab of your iTunes sync settings, select This Computer under Automatically Back Up, then click Apply at the bottom to sync. The phone will then back up to your computer every time you sync. When you get the new phone sync the phone one last time to create a new backup, then when setting up the new phone choose Restore from iTunes Backup and restore the backup you made earlier. Then restore her iCloud backup back to her phone and turn iCloud backup back to On. You may also want toperform a manual backup in iTunes to overwrite the backup of your data by clicking Back Up Now on the Summary tab of her iTunes sync settings. This will ensure that the backup represents her phone, not yours, should she ever need to restore from it.

If you are syncing your phones to different computers, you should restore her backup back to the phone, the back it up to her computer using iTunes (go to the Summary tab of her iTunes sync settings and click Back Up Now). Then you can restore your backup back to the phone and continue using it. When you get your new phone, during setup choose Restore from iCloud backup and restore the iCloud backup to the new phone. Then restore the iTunes backup back to her phone by clicking Restore Backup on the Summary tab of her iTunes sync settings. Finish by turning iCloud backup back on on her phone. Doing things this way will prevent your phones from getting each other's iTunes media on your phones if you maintain different iTunes libraries and sync to different computers.

If you haven't synced before you'll need to take some precautions the first time to avoid having iTunes delete all your apps and iTunes media from the phone the first time you do. I'm away now for a bit but will post the steps you should take when I return.

If this is your first sync and there is already data on the phone, perform the steps listed below. This will transfer your apps and other media purchased from the iTunes store to your iTunes library, perform a manual backup and restore, then sync the iTunes media back to the phone from your iTunes library. This will not transfer media from other sources such as music from your ripped CDs. As you said you never synced before, I assume all of your apps, music and other media was purchased from the iTuens and App stores. If this is correct, you won't lose anything.

1. Download and install iTunes: http://www.apple.com/itunes/.
2. Launch iTunes, go to the View menu and click to check Show Sidebar.
3. Go to iTunes>Preferences>Devices (Edit>Preferences>Devices if using Windows) and check "Prevent...from syncing automatically".
4. Go to Store>Authorize This Computer and authorize all Apple IDs used to purchase apps and other iTunes media (music, podcasts, movies, tv shows, etc.) on your phone.
5. On her phone, go to Settings>iCloud>Storage & Backup and turn iCloud Backup to Off.
6. Connect her phone to your computer and in iTunes go to File>Devices>Transfer Purchases to transfer your purchased media to your iTunes library.
7. Right-click on the name of the phone on the left side of iTunes and click "Reset Warnings".
8. Right-click on the name of the phone again and click "Back Up" to back up the phone.
9. Right-click again and choose "Restore from Backup...", select the backup you just made to restore from and restore the backup to your phone. If prompted to back up again, decline.
10. Go to the Info tab and make sure nothing is checked so you aren't syncing calendars, contacts, etc. with iTunes.
11. Go to the Summary tab and select This Computer under Automatically Back Up.
12. Check the other tabs to confirm that your sync settings are the way you want them.
13. Click Apply at the bottom to sync your phone.