Mmm Bookcase, I think you need a massive amount of commitment to raise a child with two languages, and above all you need a good reason for doing so. And more importantly, you have to convince your child that there is a good reason for having any sort of interest in that language. Kids are the ultimate pragmatists and will drop a language if they don't find it relevant to their communication needs. So how can you generate that interest?
Firstly by being interested in the language yourself, whatever it is. Be a good role model by watching videos in that language or going to language classes. Go there on holiday and let your child see you using the language (however flawed your grammar may be).
Get an au pair from that country who will speak to your child in her language - 6 months at 20 hours a week will have an enormous impact. Back this up with formal lessons from a trained teacher once the child is older.
Can you enrol in a bilingual nursery or primary school? Kids are most willing to learn a language when their buddies are using it too.
Advertise for a 'tandem' partner - a family with kids who have maybe just moved from the country where this language is spoken - offer to take them round or help them out, and when your families socialise you can use both languages. The kids will learn from each other.
Are there Saturday schools near you for this language? For instance there are a number of German Saturday schools around the country that meet once a month and provide a forum for kids to do things together in German - it's more social than real formal school, I get the feeling.
My own two children are bilingual, but it's straightforward for us because we speak one language within another linguistic culture, so the kids are naturally exposed to both. Nevertheless, we've worked hard at balancing the two. Many other parents at my daughters' bilingual schools have used the techniques I've mentioned above, though, to varying degrees of success.