I will probably soon have the same problem - I am Russian and I live in the Czech Republic. My wife is Czech (although she speaks perfect Russian too) and we have a son. He is 2+ years old at the moment and we have made the rule that she speaks to him in Czech (sometimes in Russian too, as I'm often at work and can't be with my son as often, as she can) and I only speak to him in Russian. When I ask him about something, I almost always tell him to answer in Russian - when he answers in Czech, I ask him something along the lines of "And how do you say that in Russian?" He doesn't always remember, but from time to time he does and sometimes even tries to correct either me, telling me that the correct word is the one my wife uses, or my wife, telling her that the correct word is the one I use.
We've decided that, as he grows older, I will only speak to him in Russian and we will try to make it interesting/fun for him to speak Russian too. A couple of thoughts about how to do it that we have:
- his grandparents (my parents) don't know any Czech. He loves them and I think he will want to communicate with them - and for that he will want to speak Russian;
- I myself am a linguist and have always tried to learn new languages. Haven't stopped, still learning in my spare time. I hope that, in trying to become more like his dad, he will try pick up my hobbies too - one of them being learning and speaking other languages;
- we plan to somehow explain to him that knowing other languages makes him cooler, as @Uko suggested in his answer. Or smarter. Or anything else, whatever makes him happy;
- we are trying to have another baby at the moment and I intend on giving them this idea that Russian could be a kind of a secret language for them, that no one around would understand and that they could use just for fun when they don't want people around them to understand what they are talking about (or, even more likely, what they are writing about - because of the Cyrillic alphabet);
- I am showing my son Russian cartoons and when he gets a bit older I want to show him some Russian movies that I know I liked as a child. They aren't available in Czech and I hope he likes them - that way I can from time to time show him more, and he would need Russian to understand;
- to expand the previous thought - there's also great Russian books and music that one wouldn't understand without knowing Russian.
Those aren't confirmed strategies, but something I've come up with and intend on trying to use. Hope it helps or at least gives you a successful vector of action. If I remember anything else, I'll post later.