How would you raise a child bilingually?

Say you wanted your child to be bilingual, say English and Spanish. (Of course considering you speak both languages yourself as well.) How would you be able to raise them to know the differences between each language and know when to use them and who to use them with? What would keep them from speaking "Spanglish" (A mixture of both... Example: I gusta el dog)? And how could you keep them from being more accustomed to one language than the other or forget it? I know there are some people who learn both from their time of birth/soon after; I just don't understand how they teach them?