How can I delete songs from my iPhone while keeping it intact for my iPod?

In iTunes, make a copy of the playlist that you want to use on your iPod touch. Rename it to ___ iPhone (so you know which one it is). Then delete the songs you don't want to have on the phone.

Next time you go to sync your phone, click on the Music tab, and uncheck the playlist you had before and check the new __ iPhone playlist. Then hit the sync button and you should be good to go.