A luxurious soak in the bathtub is that much better when you can sink all the way up to your chin. Use Deep Water Bath to override the overflow valve and get extra inches of water in the tub.
Home decorating blog Apartment Therapy directed us to this cool gadget. It fits over your tub's drain overflow valve to let the water rise nearly to the rim. The semi-plug has a drain hole at the top, though, so your tub won't actually overflow. It's removable, too, and easy to stash when not in use. Pick it up at Amazon for around $US7.
For a self-made and much more permanent approach, Apartment Therapy has some other ideas for filling your tub nice and high. Check out the full post for those ideas and other tips on getting a nice, deep water soak. Share your favourite bathtime luxury tips in the comments.