While a lot of adults will struggle trying to learn another language, kids can usually pick it up quite quickly. It's always surprising how fast this can happen, especially if you make a point of fostering this as much as possible, within reason. It's always important to keep things fun, doubly so if your child is quite young.
1. One Language Per Parent: This technique works well if you are already bilingual or trilingual. You make a house rule out of it. Your child will have to speak a different language to each parent. This works well when you combine English with another language that you want your child to learn. English/French, English/Spanish, and English/Mandarin are some examples.
2. One Language Per Room or Per Floor: When you use this learning tip, make it a house rule that in one room, everyone must speak the same language while in another room, they must change it. Keep the languages the same for a determined period of time, 4 to 12 months. Changing them too often will lead to confusion. In bigger houses, you can have the same rule but for different floors.
3. Movies & Media: It's easy to stock up on movies and DVDs that will allow your child to develop another language. Most DVDs now come with a few different language tracks, which will allow you child to watch a movie in another language. It's best to start out with movies that your child has already seen. If you child is old enough (9+ years old), you can put on the English subtitles for a few times before switching to the foreign language. You can also get some products like board games in a foreign language, for example Monopoly or Memory (Ravensburger makes a great one for English/German learners).
4. Classes & Tutors: Finding a tutor or introductory classes can also be an option for some children. Classes that include a lot of games (flash-card based) and some media will make learning another language a lot easier than just sitting on a chair for a whole hour. Looking through the employment boards at the nearest college and university might be a way to find tutors that are less expensive than classes at a school.
5. Immersion: There's no replacement for full immersion. This can be done in a school in your city or if you move to another country. If you're interested in French, Quebec (Canada) is near enough and the public education system is mostly French. Your child will be able to attend. There are also quite a few different immersion schools, but they tend to be located in big cities and be expensive. The Lycée Français de New York is an example of such a school.
MORE BILINGUAL TIPS
Raising a Bilingual Child
Introducing a Toddler to a Second Language
(Images: Flickr member CB and GK licensed for use under Creative Commons, Flickr member Speedboat licensed for use under Creative Commons, and LFNY)