I have played many, MANY time management games and found this game to be the most challenging ever. The biggest hint I can offer is: chains, chains, chains. This game cannot be finished without using chaining to it's fullest. Give out menus to everyone, fill all orders first, then collect money. Do this over and over as fast as you can and you can beat the hardest levels. I was stuck at level 39 for days until I discovered that chaining was critical to finishing - and once I got that, I finished level 60 the same day.
Other hints include:
Buy the sauce/coffee upgrade as soon as you can and also buy the two item table option as soon as possible. Then, put extra sauce/coffee on the table as often as you can and that helps when the multiple sauce/coffee orders come in. Try to remember to keep doing this - but doing it even just at first will help.
Learn the 'personalities' of the characters and serve those that are impatient first. There are some that will patiently wait for quite awhile while others grow very impatient very quickly. Serve them first - the others will wait.
No matter how tempting, keep chaining. Let the money sit and collect it all at once. Let people wait and give out menus all at once. Stopping just once to pick up money will effect your end total - don't do it. Just keep chaining and serve as many people as you can as fast as you can.
And no matter how many customers leave or how many items you have to toss, keep going. I finished rounds that I was sure I would lose - after multiple lost customers, tossing items, etc. - I still made it past the round. So keep going.
Unlike Cake Mania (for example), saving mistakes or making things ahead doesn't help. Chocolates left on the table disappear shortly after you put them there...so just toss and move on.
And ignore the 'customer satisfaction' goals presented in the written pages before each round - they don't matter in terms of getting to the next round.
The orders picked up by the Zeppelin should be your lowest priority. They don't effect the chain bonus as much. I would serve the Zeppelin order last, then pick up $$, then wait for the line to repopulate and give out menus. The zeppelin orders are very patient.
Good luck! It's tough but you can beat the game.