A few ideas...are you filling the bathtub with water before putting the dog in the tub? For some dogs, having their feet suddenly submerged like that can be very frightening. Rather than using a tub filled with water, place the dog in a dry tub and use either a shower wand or even just an old jug that you can fill with water and pour on the dog (that's what we do). You may even want to try some "dry-runs" (literally) where you simply put the dog in an empty tub, reward and praise her when she's calm and still, then remove her. It keeps it all very positive and can help her realize that it's not all bad.
What kind of surface is your dog standing on? Bare tub or tile? This kind of substrate is super-slippery, and can cause some dogs to panic. We use a rubber bathmat or even an old bath towel on the bottom of the tub to give our dogs some traction, which is calming to them.
Remember, too, that your dog will be feeding of your emotion. If you go into that bathroom all tense and thinking about what a nightmarish ordeal the bath will be, your dog will pick up on that and will mirror that tension and stress. Stay calm, upbeat, happy and light. Keep your actions and your movements calm and smooth.
Of our three dogs, two will tolerate the bath quietly, but one is a "screamer" (which is odd because she's such a water dog
)...as long as we don't make her stand on a slippery floor and we're quick, she isn't too bad.
Solomon - black DSH - king of kitchen raids (11)
Gracie - Mutterooski X - scary smart (9)
Jaida - GSD - tripod trainwreck and gentle soul (4)
Heidi - mugsly Boston Terrier X - she is in BIG trouble!!! (3)
Audrey - torbie - sweet as pie (11 months)
Patrick - blue - a little turd (but we like him anyways) (6 months)
Boo, our Matriarch (August 1 1992 - March 29 2011)