This is a good one. You obviously can't continue to learn more and more tricks and perform them at the same level at which you perform your best/favorite tricks. I'm not saying you can't learn more, but I am saying it will be awhile before the new ones are anywhere close to the performance level of the old ones. I am finding myself in the exact situation that you mention. It's good to have repeat customers (good for the restaurant and flattering for you knowing they returned to see your magic again).
Some restaurant magicians will say to only pack 3 or 4 tricks and repeat them throughout the night. I personally disagree with this because one, it will get boring real quick...and two, you won't seem too much like a magician if the table you just performed for, sees you perform the same exact tricks to a table near them. The though would be "man! Is that all he knows". The other issue is that you need to have a few kid friendly tricks as well (depending on the type of restaurant). At any one given time in a restaurant, I am prepared with 10-15 different effects that include coins, paper money effects, rubberbands, pens, silks, TT, Jumbo Coin, paper bag, sponge balls/rabbits, jigger (chop-cup routine), etc. And of course I have a deck of cards from which I can peform many, many effects. This might seem like a lot, but I carry what confortably fits in the various pockets between my jacket and pants pockets. This works for me but it may not be for everyone. It certainly has helped with some repeat customers who request me everytime.
Now since there are only so many kids effects that you can pack on you and perform in a walk-around restaurant atmosphere, this has started to pose a problem. I've been dealing with this recently with a few families that have come back 4 and 5 times in the past 2 months. Of course I always want to show them something they haven't seen, but there's always going to be a limit to the amount of current stuff you perform as well as new stuff you can bring in such a short time.
My recommendation would be to milk all of your effects to the maximum length of time possible (keeping it entertaining of course) and perform fewer effects for tables involving kids. This way, if they come back, you'll have other material to do. Additionally, even though kids don't always get the card and money tricks, you can start with a kid effect and then tell the parents/adults that you don't want to leave them out, so you're going to do some just for them. Then you can have a little more variety. Before you know it, the food will come and you'll be done with that table anyway.
As for learning new tricks, you're always going to have your favorites that you'll keep doing forever. However, it's always a good thing to learn new tricks and further your knowledge in magic and presentation to become a more well-rounded entertainer.
Keep in mind, these are just my thoughts and opinions. Others may not agree and others will have there own opinions. The truth is, if you get a steady restaurant gig, you'll be inclined to learn new stuff to keep from getting bored doing the same things night after night or week after week. You might think that's not possible, but it is. So for now, just master what you've got including handling, presentation, audience control, ect.