I grew up bilingual, and so does my son of 18 months. My son and I both have a Danish father and an Austrian mother. Here is what I've learned, from my own life as child and as parent, and from others:
Start immediately. it won't do to decide on this after a year or more. It must be from the start, because kids learn even before birth, and most under the age of 10 months. They need to learn the "melody" and the sounds of the language, and that only works well early on.
You must be native speaker, or equal to it. I've seen Austrian homes where one parent speaks poor English in an attempt to teach the language. Doesn't work. It's not enough to teach individual words from picture books. It's not enough to speak what you learned in school. You've got to know all the words you're ever going to need, and you only know that if you're a native or if you are immensely good at English as a foreign language. Don't even get me started on pronunciation and grammar!
Be consistent. I speak Danish to my son 99% of the time. I'd say any less than 90% and it doesn't catch on well. I do speak German with him if he's around other kids/people if I need them to understand too. But I also speak Danish to other small kids, mostly for fun but also to show that it's not a secret language.
Show, don't tell. I speak Danish to my wife 80% of the time. There are things that I can tell her easier in German, but mostly my son hears me talk this language with others too. It's not just for him. My side of the family also speaks Danish, wife's side German. Luckily we all understand each other.
Act natural. Speaking any language is normal. Don't act special when speaking any particulate language. There should be no difference in the way you act, in relation to what language you speak. They're both just a language.
Books and stuff. This is actually the difficult part! It can be hard to find enough material/books/toys in the current country's foreign language. In my case, we're surrounded by German stuff but it's an effort to ensure enough Danish material to keep the balance. In particular, media goes here -- television, DVDs, computer stuff. Plan ahead, is the best advice I can give for this point.
English is third. In our case, we're not in an English-language country. Third (and fourth, etc.) languages come easier to the child if the two primary languages are firmly in place first, so don't worry about thirds in the beginning, except if you are in an English-language country, in which case it should be mixed in naturally.
That's it from the top of my head. I might edit and add more later.