A couple of quick tips:
Now to the tips:
1. Assuming you have done your best at shooting time, then you can use the dodge tool to treat the borders. It comes quite handy for the tricky edges. You can change size and hardness of the brush to suit your needs. Once you get a hold of it, you can get those complicated edges not to look as if they have been cut with scissors. :)
Of course, dodge tool for white backgrounds and burn tool for black.
2. To make sure every last bit of background is completely white (otherwise you might have your images rejected) use an adjustment layer to over saturate your image. Get that saturation slider to 100%. Of course it will look awful but you will be sure that you won't miss a bit of background. After you are done, just delete the layer and go have a cup of coffee and play with your kids for a while before doing any more work. Your eyes will be all messed up and you will make mistakes related to white balance, hue and saturation. They will need a short break.
So I hope this hasn't all been said before and that you find it useful.