http://support.apple.com/kb/HT1495

Hope this helps!
References :

This means you’ll share the same computer and same iTunes account, but we’ll set it up so that ONLY song you want appear in the playlists.

Step 1: Create a Playlist.

- Start iTunes. Select ‘File’ from the toolbar and select ‘New Playlist’.
- Give it a name : "Mum’s Songs" will do for now.

Step 2: Add songs to the new playlist.

- Select ‘Music’ under the ‘Library’ heading in the left hand menu.
- Right Click on a song and select ‘Add to playlist’ then choose "Mum’s Songs".
- Repeat on all of the songs you want on YOUR iPod.

Step 3: Tell iTunes which playlists to include on YOUR iPod.

- Plug in your iPod.
- Select your iPod in the left hand menu under the ‘DEVICES’ heading.

You’ll now be on the summary page.

- Select the ‘Music’ tab.

Okay. Here you’ll select the playlists that you want to appear on your iPod.

- Tick the box that says ‘Sync Selected Playlists’
- Scroll through the various playlists and tick the ‘Mum’s Songs’ box.

- Now hit the ‘Apply’ Button at the bottom.
- Next hit the ‘Sync’ button at the bottom.

All done!

Now whenever YOU buy a song, just remember to do step 2 again to add the new song to your "Mum’s Songs".

Last note. You can have multiple playlists on YOUR ipod. Just be sure to tick them as you did in step 3 for "Mum’s Songs". This is cool if you want to set up "Mum’s Rock" – "Mum’s Faves" etc etc.

Have fun!
References :
http://www.doipod.com