You have probably experienced "boneless leg syndrome." This is when a dog, (almost always a large one!) is convinced that it has no bones in its legs. The poor creature simply cannot stand up while in the bathtub or on the grooming table. Repeatedly lifting these dogs up is a recipe for straining your muscles.
I have found that if I place a solid object, (that is a 5 gallon bucket in the above photo,) padded with a towel, gently under the dogs sternum they miraculously remember that they do, indeed, have leg bones and they really can stand up!
You will find 100 more helpful hints like this in my book, Practical Grooming Tips. For more information go to: http://www.darylconner.com/Book1.htm