[Gerald]

When done the right way, a serious look can look sexy and sultry. However, if you fail to pull that off you just look like your dog died. That's up to the photographer to see before they take the shot.
Some ladies can also be a bit head-strong on what they "think" looks good and guys usually lie about it just to earn brownie points and avoid castration.

I prefer a coy mischievous smile that says "I'm going to have fun with you".
A little less Russian Ballerina and a little more Daisy Duke makes a great ad.