A 40th birthday party can be an elegant affair or it can be a throwback to college days. Rather than taking an "anything goes" approach to your birthday party, you can choose a special theme and require guests to dress up for the occasion. Try to choose a theme that your guests will enjoy and that they can easily find costumes for.
Dressing up for a 40th birthday party doesn't have to be wacky and wild. You can choose to throw a black tie party. Guests should dress in formal wear, with men wearing tuxedos and women wearing evening gowns.
Choose a specific era and ask guests to dress in clothes from that time period. This can be as far back as the Renaissance period or just throwing an '80s party. One way to do this is to ask people to dress in the style from the year that the birthday boy was born or the year that he graduated from high school.
Guests can interpret this however they see fit. Men may want to wear simple suits or dress in a flash zoot suit. Women can wear a short cocktail dress or perhaps a flapper costume.
For a garden party, men can wear linen suits or go even more casual with khaki trousers and button-down shirts. Women can wear a floral dress with a large sunhat to protect their skin.
A Western theme can be a good theme for those that enjoy country music. You can interpret this to mean dressing in modern Western wear, with jeans, flannel shirts, cowboy hats and boots. However, you can also interpret this as period Western wear, with frilly dresses for women and chaps and gun holsters for men.
A toga party appeals to many because it's an easy costume--all you need is a sheet to wrap around your body. This can also remind people of the college years.