Before you work at any restaurant you need to answer a couple of questions. Do you want to book more gigs from this restaurant? If you do, then you need to know what kind of gigs you want (you wouldn't work at Joe's Crab Shack and expect people like Michael Jackson to walk in, etc.). Do you want this restaurant so you can constantly have real time practice? In this case, you wouldn't argue over price and you wouldn't mind performing for free.
I'm sure you have heard this before but you need to sell the benefits, not the features. Tell the General Manager what you can do for the restaurant and what you bring to the table. As a magician, you can entertain the people who have to wait for 30min or more. You can act as a mediator for someone who isn't happy with the service (ex. food takes too long). You can be the 'extra something' (at no cost, therefore adding value) for people who reserve a special area in the restaurant. You can make the customers feel better which in turn leads to better tips for the waiters. You are an extension of the restaurant and the embodiment of a magical atmosphere. The General Manager needs to be sold on why he is better WITH YOU at the restaurant rather than without you. He is comfortable with the way things are now so WHY SHOULD HE SPEND MONEY ON YOU? Why should he risk his neck to have a magic guy perform for people who don't expect a magician? But don't just tell him why, give him a free night to allow him to see what you do.
Above all, be creative. When you meet the GM, make sure he knows that you respect his time by acknowledging how busy he is. Set up an appointment and do research before you meet. Figure out if they had a magician/balloon artist/clown/etc. If they did, figure out why s/he isn't working there anymore. Know the GM's name and use it when you talk with him/her. And if you really want the GM to take you seriously, get insurance and tell him that you have insurance. You will get turned down based solely on the fact that the company doesn't want to take on the risk of having a magician who might upset a customer and cause that customer to sue.
The number one thing that will get you the gig is to ask questions. Questions allow you to sell. But the questions need to ask make the GM respond in terms of his/her own interests and answer in terms of yours. An example would be..."How do your customers react to an hour wait period for their tables and have you ever considered a new way to keep your customers happy while they wait to be seated?" Open ended questions are the key. Close ended questions are questions that get a yes or no response...don't ask these lol. If you ask, "Would you consider hiring a magician to entertain your customers"...and the GM says "no"...then you're stuck and you look pretty stupid.
All in all, find out why you want to work at the restaurant. Sell the benefits, not the features. Make your approach creative. And ask questions that make the GM find out for himself why you are worth it.