Where to start? Well, everyone else has already said the main things - I am a militant bilingual parent and think that anyone who doesn't speak their mother tongue to the fruit of their loins should be severely punished. Especially if they don't have a good command of the language they are using in its place.
My children were raised speaking English and German, and they are carrying on the tradition. Having two languages from the start has made it easier for them to pick up other languages and they all have a working knowledge of French and Spanish and, thanks to Erasmus, one even mastered quite a bit of Turkish.
In the early 80s, when they were little, I was fortunate enough to find half a dozen other expat mothers in my area and we started a playgroup. We found a church hall and once a week, taking it in turns to plan the morning, we played traditional games, read stories, sung nursery rhymes and did different activities to cover all sorts of vocabulary.
Although with DS1 it took until he was 16 before he actually replied in English to me, not until any of my DS married did I speak a word of German to them - and now I only do so if I am addressing them and their wives as a couple. (I have always spoken to the DiLs in German.)
The DGS, of course, I speak English with.
What I find impossible is to speak to someone in a different language from the one I am used to. For example, I couldn't speak to my DH in English. It would be like speaking to someone else.
One of our playgroup had an Italian husband and the children managed the three languages easily.