I have not done this, but we have friends who are raising their daughter to be trilingual. The mom is French and the dad is Columbian. Sofia's mom speaks to her only in French, and her dad speaks to her only in Spanish. She goes to daycare where she is only spoken to in English. The daycare is also teaching her baby signs, and her mom told me that this seemed to really help her tie all three languages together. Sofia is one and a half now and has very few words, but some language delay is really normal for kids raised w. multiple languages. Sofia's mom told me that she did have to educate their doctor about this, b.c he was all concerned w. her lack of words at 18 months.
I have a friend who inadvertently did this also- they spoke only Punjabi at home and planned that their son would learn English when he started school. But then he picked up Spanish from their housekeeper as well.
The school that my son is districted for just turned into an immersion Spanish or Mandarin school. Some of our friends went to immersion schools like this and still speak the second language fluently, so this is something we may do in four or five years when he is old enough.
I grew up bilingual. My father spoke to me in English, my mother in Norwegian, and when we were all together we spoke Norwegian when we lived in France, English when we lived in Norway, and Norwegian when we lived in England. In addition to that we always spoke French on Saturday's, and Italian on Friday's (but this started when we were older). Clear rules as to who speaks what is essential, otherwise your child will be confused. You must be prepared that they most likely will start speaking later than other children.
What we are doing with Roo; I speak Norwegian to her when it's just the two of us. As a family we speak English, and boyfriend speaks English to her, but sometimes sings her lullabies and read night stories to her in Czech (he doesn't speak it fluently enough to teach it to her). The fascinating thing is how two languages connects the "wires" in your brain, it becomes much easier to learn new languages, children who's spoken or been talked to in two or more languages from they were small has a much easier time picking up languages as they get older (I was talked to in two-three when I was little, now I know 6 (7)).
The first thing I will say; don't speak to your child in a language you're not fluent in. And it needs to be very structured, otherwise your child will not understand, and the whole thing will be a waste.
I don't have kids, but I'm speaking from experiences with nieces and nephews. However you decide to do it, make sure you stick to rules. My niece would latch on to a word in either English or Spanish, then refuse to use the other word. She would get really confused- water was "agua" and there was no convincing her otherwise. We told her "agua" and "water" were the same thing, but she would have none of it. It got to be difficult if she was speaking with people who didn't understand the other language, because she just couldn't accept that there were two words for everything. Similar problems arose with my nephews. I think if you do your research and really stick to a system, you can probably minimize those kinds of problems. Raising a child bilingually is a great thing to do- I always wish my parents had been able to do it for me!
We have our plan mapped out. We have/intend on teaching basic sign language starting at 6 months, which apparently helps bridge languages later. I speak Spanish fluently, so I will mostly be speaking to the girls in Spanish and SO will speak to them in English. We will continue using sign language to help them associate the idea with both words. Example, 'milk' and 'leche' mean the same thing. SO practices his limited Spanish quite a bit also, so as they gets older, we'll all use both languages more freely.
When I was a baby, my mother didn't speak English yet, so she only spoke to me in Spanish, and my father spoke to me in English. So, naturally, I was able to differentiate between the languages and communicate equally with my spanish speaking relatives as well as the english speaking ones. As my mother learned English, she used it more in the household, but continued to mostly speak to us in Spanish. When I was 10, we lived in Mexico for 6 months and I attended an international school which helped solidify my bilingual-ness.
We also have plans to live abroad at some point in the future. I'm thinking Spain? :)
My husband speaks French to our daughter quite regularly. She hasn't completely grasped it yet, but its interesting to watch. He'll ask her a question in French, and she'll respond in English. So she clearly understands what he's saying, she just isn't capable of making sentences of her own yet. We'll continue this when our next baby arrives, we started this when Amelie was very young.
I also speak French but nowhere near as fluently as my husband, and we both learned different dialects, so we were getting confused. We decided I'd just stick to English to avoid confusing her.