Aside from the obvious food and water, mother cats at this point tend to move their kittens around a lot. Provide a small, warm, dry, cozy soft secure shelter (or two) and she will likely use it. (low traffic hideaway type areas) Don't try and force it though, she will put them where she wants to put them no matter what. Only intervene in the case of dangers a cat just cannot see. (fibreglass, gas & oil waste.. etc) Also, I hate to say it but a frequently changed litterbox in a dry and very secure location might draw her attention too (nursing mother cats consume the kittens waste and the result is that hers is particularly foul - the scent is a security liability). For mother cats it is all about security.