Existing Music Deleted When Manually Synching New Music. How Do I Prevent This?

Question

I want to add a playlist in iTunes on my computer, to my iPod Touch, which has (had) around 1,200 songs on it at present.  I go through the synching process, checking "Selected playlists, artists, albums and genres."  I check off the playlist I want to add and then click on Apply.
When the process is completed, the new Playlist is installed on my iPod, and all of the songs that were previously there are flipping GONE. 
I have been wrestling with this problem for a long time, and have as yet to solve it.  I don't want to wipe out my existing songs/playlists on my device.  I want them to STAY RIGHT THERE.  All I want to do is ADD SOMETHING NEW to them.  I know this can be done, but cannot for the life of me figure out how to do it.
I have turned off Automatic Synch in iTunes.  Makes no difference.  I am only using one device and one computer.
Help!!!

Answer 1

"Is the idea that you set up a playlist (I will call it Synch Playlist) into which you will be putting only new music not previously installed on your device and you will then synch to your device only what is in the synch playlist and this will not delete any of the previously existing music on your device?"
Not exactly - this playlist should contain all the music that you want to be on your iPod - think of it as a "mirror" of what you want to be on the device.  To add music to the iPod, add it to the playlist then sync.  To remove music from the playlist, remove it from the playlist then sync.
Here's an example that may help.  I have a playlist called (somewhat unimaginatively) "8Gb iPod" which I use to control the content of my 8Gb nano.  I remove any previous content from this playlist and add five albums to it as follows:
I sync the iPod and it will contain these five albums (any previous content having been removed).  Now suppose I want to add a couple more albums to the iPod.  I add these to the playlist so that it looks like this:
and sync the iPod again.  All seven albums will now on the iPod - the initial five will be untouched because they are in the playlist and were already on the iPod; just the two new ones will be added to it.
Now I decide that I want to remove two of the original albums from the iPod.  I remove these from the playlist so that it is like this:
When I next sync the iPod it will delete the Sinatra and U2 albums, leaving the other five untouched.
With this method iTunes is essentially comparing the content of your iDevice with that of the playlist(s) used as the basis for sync operations, and adjusting the content of the iDevice to match that of the playlist(s) at sync time.
So, if I was to attempt to write down the operations that iTunes goes through for each of the three syncs described above:
First sync
remove any music that is on the iPod that is not contained in the "8Gb iPod" playlist
add the songs that comprise the "Reflektor", "Sinatra at the Sands", "Crime of the Century", "Songs of Innocence" and "Down Where the Spirit Meets the Bone" albums to the iPod
Second sync:
leave the songs that comprise the "Reflektor", "Sinatra at the Sands", "Crime of the Century", "Songs of Innocence" and "Down Where the Spirit Meets the Bone" albums on the iPod (as they are still in the "8Gb iPod" playlist)
add the songs that comprise the "Independant Intavenshan: The Island Anthology" and "The Endless River" albums to the iPod
Third sync:
leave the songs that comprise the "Reflektor", "Independant Intavenshan: The Island Anthology", "The Endless River", "Crime of the Century" and "Down Where the Spirit Meets the Bone" albums on the iPod (as they are still in the "8Gb iPod" playlist)
delete the songs that comprise the "Sinatra at the Sands" and "Songs of Innocence" albums from the iPod (as they are not in the "8Gb iPod" playlist)
Is this clearer?