Some people will actually try to dress for the event they are going to. (Like if a wedding is outdoors, you want to
try and wear something cool, not black because you look good in black.)
Other than that, without specifically telling people what to wear the closest you can do is actually say on the invitation the type of formal wear it would be.
The closest I can think of for your event is something along the lines of smart casual. (which does give space for your female guest to wear slacks if they aren't dress type).
That way you are at least giving them an expectation to try and put some effort in...at the same time not alienating folks who may not want to wear a dress, or for the men khakis.
http://en.wikipedia.org/wiki/Smart_casual - Because if its on wiki, it must be true.
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