Wow this seems to be my most popular question. I've answered it several times. I love how many guys want their wives to be sluts and how many wives who have asked how to be more of a slut.
First never push her. But always encourage her telling her how beautiful she is and how much it turns you on to see her being slutty. Buy her outfits you would like to see her wear. On the beach when she is sunbathing encourage her to let you remove her top and even her bottoms as she is on her stomach getting sun on her back. Hopefully she will roll over exposing her front side as well.
When your buddies are over encourage her to dress slutty and maybe join you for strip poker hopefully allowing your buddies to not only enjoy her nudity but hopefully become physical with her as well.
Post pictures and videos all over the web (with her permission) exposing her for all to enjoy. This works on her mind that if all these guys like her on the web posting dirty messages even masturbating to her then just maybe she will be more open to doing it for real.
When outdoors in secluded places encourage her to go naked and get some sun. In time hopefully it will become more public. One great place is in the dunes at the beach where it is always possible for someone to see her.
The list is endless. Email me directly at email@example.com and we can discuss it further or even better have her email me and maybe in time with a little encouragement she will be more the slut you desire. Good luck to you both.