Ok, tricking your wife behind her back isn't going to solve anything. First she's going to wonder why she's not getting pregnant eventhough she's draining you every chance she gets. Next she's going to think something is wrong with either you or her, and will go into some kind of funk. That's when she'll want to call in a doctor to check you and her out. When they find nothing is wrong, she'll look back at you and you'll tell her about your grand scheme not to knock her up.
I say the easiest and quickest solution is to sit down and talk to her. Tell her why you don't want kids and listen to the real reason she wants another. I don't mean listen to the first reason she says because they're usually lying the first go round; get her to tell you the real reason. More than likely she's bored and it could be solved by you being more romantic with her. In fact, I think that will fix it since she wants to have YOUR kid.