Probably the easiest way is just to create a new database, unless you really do want to mix demo data with real data.

Start up a second browser (eg, if you're using FireFox, then also use Chrome), connect to OpenERP and create a new database with demo data.

Switching back and forth between the two is then as easy as <alt><tab>

I have over a dozen databases created on my development server with no negative effects.