Just be emphatic.
If you were sitting in a table , what did you expect for a performer?
Maybe you are with your wife, and you want to talk important bits, or just with your friends searching for some fun.
BE open. You just need to chat a little bit with them. In that way you dont present yourself as a "magician" , "mentalist" or whatever. You are just giving a good moment, asking for the food or just sharing a smile.
If you see expectation from your possible audience, you introduce yourself as a performer and perform 1 effect.
Yes... one...
NOW ... if you see more interest, you can keep going with your routine. BE flexible. Maybe your first effect blow the mind of your audience and they want to see more, or maybe not. IS fine.
As I told you, you need to be awared that each table is different.
If you work in a restaurant,be aware that childrens can be an important factor. Can be incredible trouble maker, or if you use them properly, can give you a lot of contracts with his parents.
I perform mentalism , so a simple visual routine of choices, can give the kids a nice moment , and turn the red light for the parents.
Another tip. NEVER approach a table with a prop ( cards, coins, etc.) That communicates that you protect with your prop and also communicates that you arent sure about your approaching and sharing.
Just be yourself, expand your natural entertainer abilities and just have fun. If you have a good time, they will respond in a nice way.
Alcohol can be other factor. For that reason just perform always 1 effect AND if you see that you can perform more, do maximum 3 effects, 5-10 minutes max.
If your audience is more interested, just give him your business card and leave them wanting more. In that way you can give them a private show , charging more

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There´s a LOT of tips that I can keep writing, but the BEST way to learn is just go and do it.