How to use minecraft snapshots?

I went to use the latest snapshot so I can play with the new blocks and what not. After playing on single player for a couple of hours (with the new snapshot) I went to play on a multiplayer server. When I went to click on the server it said it was outdated and need version 1.7.4 so I went back to the menu and clicked on 1.7.4. After that I was able to play on the sever and everything was okay. But then when I went back to my single player world all the items that I collected was gone. What happen? How do i play single player with the latest 'snapshots' and play on the 1.7.4 server?