When I'm heading out for the night, but still want to be mainly casual and comfortable (which is pretty much how I operate), I pull a few simple switches to my denim look from the day. Replace tee with flowy, silky top; add more jewelry; switch out flats for pumps-boom I'm done. Simple and easy, and I still feel appropriate for dinner, drinks and a movie.
Top: Marshalls (similar)
Girlfriend Denim: Gap
Heels: Rockport (similar)
Ring: Sorrelli - I just found this ring that I forgot I had and I'm in love all over again ;)
Polish: Butter London "Kerfuffle"