Raising Kids to be Bilingual

I have one son who is almost 3, and he is learning Spanish and English.  I had the fortune of working as an English teacher for adults for the first 3 and a half years that I was living in Mexico, and I asked a few more experienced (and better) English teachers for tips on teaching young children 2 languages.  The approach my wife and I chose was that one would speak only Spanish (my wife who is Mexican) and the other English (myself.)  Despite our best intentions, we've made a few careless mistakes:

• Teaching the wrong word first, because it seemed easier for him to say
• My wife taught him "meme" for "sleep" and I taught him "yum yum" for "eat"
• We are now paying for this, fighting uphill to convince him that those words are not correct
• He can say "sleep," "dormir," "eat," and "comer" but he has tended to stick with the first, wrong words we taught him ("yum, yum" has finally been replaced with "eat," but it took a good deal of persistence)

• Accepting Spanglish combinations instead of insisting on attempts at only one language
• This one's tough; we want to be praise him when he says new words, but after we let him get away with saying "yo eat" once, that's they way he always says "I want to eat" now, even though he's capable of saying this almost correctly in both English and Spanish

To make this all more fun, he throws in some of his own inventions; "bo, bo" means something is falling or hitting another object; "ow" means something is broken has come apart.  So, when he tells the neighbour "yo ball bo, bo. Bus ow," she just looks at us with a look on her face saying, "I know all of that means something, but I really have no idea what."  (Translation: "I threw the ball, which hit the [toy] bus and broke it.")  He likes to re-enact, so this helps at times.

Recently I've started the habit of saying "I don't understand 'agua'," cuing him to repeat the phrase saying "water" instead (and my wife the same in Spanish.)  Now and then, when he's being stubborn, I have to lead him into saying the English word like this:

• Me: Who says "agua"?     Son: Mom!
• Me: Who says "water"?   Son: Dad!
• Me: What does mom say?    Son: Agua.
• Me: What does dad say?    Son: Water.
• Me: So, what did you want?  Son: Water.

 I don't do this too often, since translating word for word is also a bad habit.  Sometimes he makes a game of it, giving the wrong answer, and getting a mischievous smile on his face.

He knows that when a question is in English, he answers "yeah" and when it's in Spanish he answers "si" (without hesitation or cues.) Little by little, he's getting the difference.  He has also started a bilingual pre-school which seems to be helping.  I suppose I should read more about this.

If you've got any suggestions, feel free to share.