If the vet's ruled out all medical conditions that could be causing a dull coat, I'd first look at the food that's getting fed. A lustrous, shiny coat starts from the inside. Depending on what you're feeding, it might be helpful to upgrade kibble to a better quality (fromms, orijen, evo, innova, etc) to help get the coat in better condition.
What shampoo do you use? Get a high quality shampoo made especially for dogs. I am a HUGE fan of earthbath. Here's the link: Pints :: earthbath® - totally natural pet care
and you check the shampoos to see what might be best for your dog. I love the mango tango and puppy shampoo, myself and have bathed my dog with them about once a week this summer - it's actually improved the condition of his coat and skin. When you bathe, make sure you lather, rinse, repeat...and then rinse for longer than you think to make sure all the soap is out. I usually go another 3 minutes of rinsing past the point where I think I've gotten all the soap out.
Along with the bath, if you're brushing her 5 times a week, you can spray her coat with a conditioning spray. Deodorizing Spritz :: earthbath® - totally natural pet care
(I swear I'm not affiliated with Earthbath in any way - their products are just amazing!!) I have the mango tango spray and Ranger gets it sprayed on and hand-rubbed in down to his skin before every brushing.
For additional dietary supplement, a raw egg (shell and all) a few times a week is greatly beneficial for a coat. You can also get salmon oil pills or liquid and give those on top of the food. If you go with the pills, pierce the capsules and squeeze out the oil onto her food. If she doesn't pop the capsules before swallowing them, she won't get the full benefit of the oil. I give Ranger 3 human grade salmon/fish oil pills a day - about 1000 mg each - we live in a super dry climate and he gets really itchy without him. I just give them to him as a treat since he pops the capsules - you can hear it pop in his mouth before he swallows.
Also, I have no idea how much she weighs or anything but if she's already on a high quality food, she could need some digestive enzymes/probiotics to help her get all the nutritional benefits from her food. I know a guy who's golden has to get powdered enzymes/probiotics otherwise she loses weight and her coat condition gets dull - and she's on Evo. If she has her digestive enzymes, she looks great. So that's a thought...
It will take a few weeks or months to start seeing results if you decide to change her food, add supplements and change "beauty" products so be patient. She should have a gorgeous coat soon!