How to do an incremental sync

If you are currently syncing All music, only the new content will be added. If you're syncing selected playlists, then the new content will be added if you first add it to a synced playlist, or create a new playlist and select that for syncing. Either way, only music not already on the iPhone is going to be added during the sync (i.e. every sync is an 'incremental' sync, as you describe, unless you either Restore the phone or connect to a computer you don't normally sync to, since the iPhone only syncs with one iTunes library).

It should not re-sync everything, but you must leave everything checked that you want on the iphone.

You can check whatever you like to sync, playlists, artists, albums, etc.

The iphone will only have the checked items, so if you uncheck anything then it will be removed from the iphone.

If you have transferred iTunes content from your iTunes library on your computer to your iPhone via the iTunes sync process previously, one of these options should already be selected. If so, it is incremental. If you had "Sync All" selected, only the new music added to your iTunes library on your computer since the last sync will be transferred to your iPhone. If you choose selected playlists, genres, artists, any changes made to your iTunes library in regards to your selections since the last sync will be transferred to your iphone.

If you had manually manage music and videos selected under the Summary tab for your iPhone sync preferences previously and are now wanting to sync this content instead, all iTunes content on your iPhone will be erased first when making this change.

then it removed all my music (according to the bar at the bottom of the itunes window) and added it all again.

This occurs if you sync to a different computer - by design the iPhone only syncs to one iTunes library.

If you just add new songs to the library, and it's the same computer you usually sync to, only the new songs will be put on the phone. For example, over the weekend I put music from 4 new CDs onto my computer. When I synced today, just those ~70 songs were transferred to the phone; nothing was done to/with the 1500 songs already on the phone.

Just plug in your iphone and sync.

If you have selected the entire library to sync, then it will sync whatever is new to the iphone.

Have you transferred music from your iTunes library on this computer to your iPhone previously, or was this the first time?

In regards to iTunes content, an iPhone can be synced with an iTunes library on a single computer only. When transferring iTunes content from another computer, all iTunes content that was transferred from a different computer will be erased from the iPhone first.

This sounds like what you've done with your iPhone.

I just want it to go song 1 is on the library and phone - leave it alone

Song 2 is only in the library - add it to the phone.

This is exactly how it is supposed to work and does work.

roaminggnome said it best - "Just plug in your iphone and sync."

It doesn't matter which screen you hit Sync on. All the responses you've had describe what should happen, but it does sometimes go a bit wrong. A while back I had a situation where, every time I synced, the same three songs that had been originally synced ages ago always copied over again in each sync. Not much of a problem with only three songs, but some other users reported the same thing happening but with, like, half or more of their entire library. Eventually it just stopped doing it - I never did get to the bottom of it. Hopefully yours is just a temporary problem.