I think Camilla should dress like Princess Alexandra of Kent.
Her evening gowns:
The lace should come down to touch the hip and not just on the bosoms or halfway up the bosoms.
Here is Alexandra in a lovely purple number. Although I am not a fan of the lace on this dress, the construction of the dress is perfect including the collar.
Royal Wedding - Pre-Wedding Dinner - Pictures - Zimbio
VS Camilla's
The lace on this dress is nice but it should have continued past the waist and the skirt of the dress should have flowed like the above example of PA.
Camilla Parker Bowles Pictures - The State Opening of Parliament in London 3 - Zimbio
An improvement over the Parliament dress but the lace need to be longer and the collar need to cover more of her shoulders and the sleeves should also be longer.
Prince Charles and Camilla Parker Bowles Visit Sri Lanka: Day 2 - Pictures - Zimbio