Hi! I'm a mother of a trilingual child and have another yet to come. Each parent speak our own language to my DD, and then she is in a English public school. She mixes them up a lot, but the fact is she speaks and communicates perfectly in all three languages.
I am very steady and coherent in correcting her when she keeps saying it wrong, explaining the differences in the two native languages (Spanish and Portuguese - that are very alike) and many times she is the one that corrects herself after saying it wrong... Of course sometimes I just let her be, thinking she will eventually speak well every language in due time if she needs them for work or study. Yet what I am very, very, very strict about is so she does not speak English to us, and we never ever speak in English with her (except if we are with friends and everybody needs to understand, but then is automatic changing back to Spanish once I look at her)...
I think once she grows up, she will need some support as Thespanishteacher says, specially to write properly. But in the meantime, we read a lot of books in both native languages hoping many things will stick to her through the stories and we try to see national TVs and cartoons so she hears the native languages permanently.
I was raised as bilingual child myself and my mother was always very educational about the second language, and it worked, so I guess I try to do the same.
I feel that repetition and close contact are the best practice, and don't really believe in the theories that defend children should not be contradicted or corrected, or they can get frustrated if we insist in the native languages... This is my point of view from our experience, but I guess every child is an island. I have another child coming and it will start from scratch in England, so let's see how it works with the native languages... For the first years we'll make sure it hears a lot of our languages and let's hope it works well.
Raising bi-tri-lingual might be an extra effort, but is well worth it. I believe languages are one of the best tools for the future we can give our children... Besides, that, as maths and music (both languages in their own way) develop some logical and analytical brain capabilities, different languages are also give us some great logical skills.
One thing is certain, though, they will speak the native languages with a very strong English accent! Anyway, if they ever need to live or work at the native countries, that will be the least of their problems and it will fade out with time. Writing properly, on the other hand, is a whole different issue...
Anyway, how much we laugh with the words she invents from her language cocktails... I would not give that up for nothing! :D