It is necessary to make up one's mind about the levels of bilingualism one is aiming at. The proficiency the children will gain depends not only on the language skills of the parents in the two languages involved but on the chosen method and the resolution of the parents as well. Lack of clear objectives might result in an inconsistent or misleading strategy.
There are various ways to encourage and maintain the bilingualism in the family, and there is no way of giving a final judgement about the best one to use. Though many methods are known to lead to the intended results, but they are hard to compare, due to the lack of proper statistical data.
Personnaly, we stick to the so called OPOL approach (One Person One Language), i.e. each parent speaks a different (and always the same) language to the children. In our case, I speak English to the children whereas my wife uses solely German.