there is a thread already on Diner Dash with a lot of tips and hints lol..But I can help you now
I was stuck for a while too on level 9 then I figured out if you have the podium you can seat 3-4 tables and stand at the podium to keep everyone else happy while the tables eat..instead of clicking on the tables twice to server the checks and to clear away the plates click once on each table to serve all checks then clear away the tables..this will help increase your points too and its a sure way to always reach your goal...Any more questions I can answer you can look for the other Diner Dash thread here in the general gaming forum
hope I have helped some
Found the link to the other thread that has a ton of tips in it
http://www.fileforums.com/showthread.php?t=67473