If your audience has been told of all the effects you can do before you perform them, what do you do? I know the rule of magic is never spoil the surprise of an effect, so what happens if other people do?

It depends on the effect and the routine you've constructed around it. "Never repeat an effect" is good advice for beginners, but sometimes an effect is so strong that it demands repeating. Adding a few multiple phase routines to your repertoire is a good start.

Why would your audience be told all the effects in advance? Who would know what you are going to do other than you? Even if you are expected to provide a program of some sort, you can name the effects in such a way that it does not give away the effect.
If you patter begins "I'm going to make this ball disappear, then re-appear over there" Then you need to change your patter.
If you have someone who is telling everybody "hey now he is going to ...." stop performing.

With some tricks, even if someone describes what happens in the trick it can still look magical.
Take Vernon's 'Twisting the Aces' for example.
The cards magically turning one at a time can still bring a smile even if it has been described beforehand.

I can think of any number of reasons why a particular audience might be told the 'effect' of your various routines. The most obvious being that friends who have seen what you do and were impressed have told other friends, etc. I am assuming that this is what has happened.

There's really no 'cure' for this. If all you do is perform for folks you know, and there's nothing wrong with that, then you do have the possibility of this happening. Especially, if your circle of friends is close.

The best way to handle this is to ration out what you do AND...just let time go by before you do anything for the others ... IF you really think it will make a difference. Frankly, it's good practice to attempt to fool folks with stuff they may be somewhat aware of.

Also....don't overlook the opportunity to come up with differing methods AND effects that are 'kinda like' what your friends have broadcast. This particular performing situation is a an excellent time to really think out what you do and try to find the possible 'holes' in method AND presentation. The fact that folks have been given a heads up is not of itself a bad thing. Happens all the time in magic if you think about it. Consider that magic posters used to tell outright what was going to happen at the show: Girl Cut in Half, etc.

If your technique is strong enough in almost 100% of cases you'll be fine. But, it's a good way to find out where you may have a weakness, etc. Good luck,

Be entertaining. Of course I agree it's best if they really don't know what you are going to do, but as long as you're entertaining your audience won't have the time to remember they already know what's going to happen. And if they do remember, at least they're entertained by your performance. Which I think is the main purpose anyway.

LOL.
I once had someone ask me for a program for my magic show. I asked why and they said they had to put it on their program. I said just put magic show and my name. They said they want the progression of the effects and how long each one will take and I said no. They said of I don't do it they are not going to use my services and I said "then I am sorry that I can't help you". I still have the program with my name next to "magic show".
The reason I say this is although it is funny (to me) there has been other instances where I did shows in the theatre and the reason they asked was for lighting cues. They just didn't ask it directly. I never give "program".
Back to the original question though.
If the plot of the effect is foretold it does not necesary ruin the effect. It really depends on how much of the trick depends on the surprize ending. If you are going to do a poker effect the ending can be known and still be excellent. The same with for example a cut and restored rope. There are other where it is ruined and then I wouldn't do those effects.
If you ahve a narrator then ask him to keep quiet, politely. Depending on who it is you could do something humorous even. I can think of many examples for this well.
I worked at the same restaurant for many years and sometimes people would come in specifically to see some magic. On the odd occasion I had someone narrate exactly what was going to happen. On one such an occasion (I get the wrong card and it changes in the person's hand) I went through the entire effect with the idea of palming off the card and producing it from my pocket instead of the usual ending because it would have been impossible. They had ordered some cake and when the waitress brought it I took it from her and slipped the card under the plate and putt it down in front of the person and continued (obviously having left the card under the plate) I went through the entire effect and the card changed to the wrong card. I then said "don't blame me. He narrated the effect wrong. What did you pick...oh the five of clubs well then your card is under your plate" It was still entertaining (a little at least) and he never narrated my show again.
The idea really was to get it wrong and right and say in a nice way to not narrate everything, it just worked out that much better.

Another question for the Pros, if you work at the same place all the time, wont the customers see the same tricks all the time? And work out the secrets?

There are many people coming through a place. I think the same people will not always come back. I am not a pro, just an observer.