It's been awhile since I played th game so I'm sorry if I leave anything out... I played and won both Diner Dash 1 and 2 so I hope this is helpful.....
First of all, a lot of people ignore color benefits. If you're stuck on a level, and ecpesially the las levels of the game, USE THEM TO YOUR ADVANTAGE!!!
Second, chain EVERYTHING!!! For those of you who don't know how to chain, all you have to do is seat, give menus, give food, give tips, clean dishes. DO NOT BREAK YOUR CHAINS!!! Chains can be the sole thing that gets you through a level!!!!
Third, be fast. This is probaly the simplest part of the game. Find out ways that work for YOU to help speed things up.
Fourth, just stand at the podium talking to everyone until the end of the shift. then seat people 3 to 4 tables. In between serving the people at the tables, talk to the people at the podium.
Fifth, if you follow tip number four, then use this one too. When seating the customers, seat them in groups that go together. For examples, seat the senior citizens and college guys together. Although you might be tempted to seat college guys and business women together because they are both impatient, DO NOT EVER DO THIS!!! NEVER!!! Doing that will be like a death sentence to your game!!!!! Seat customers with matching personalities together.
Again, sorry if I left anything out

like I said, its been awhile since I played the game, so I might have let a few things out. Feel free to add anything on.