I always think it's so much easier to look glamorous in the summer.
I think a good false tan is a must, as it always makes you look and feel good. A dress which is a good cut in a colour that really flatters your complexion would be great (you might be able to pick something up in the sale at Monsoon). A stunning necklace to compliment the outfit and the best shoes you can afford, and you're away!!
(I would stick with a fairly plain dress if I was going to go for an elaborate piece if jewellery though)