I had incredible success with carnations last year (try Chaubaud variety). Zinnias are great and quick (about 60 days from sowing to bloom). Sweet peas can be direct sown now (or sow outside in containers like winter sowers if you're not yet sure where they'll go in your garden). Baby's breath is a beautiful filler.

Check this other thread for more ideas.

http://forums.gardenweb.com/forums/load/cutting/msg112350129433.html

I would also recommend sweet william and garden asters..the taller varieties. SW is wonderfully fragrant and both last a couple of weeks as cut flowers.