People seem to abuse the word "dapper" quite a bit. I hear it applied nearly every day to things that are far from actually dapper. Here is some clarification. Dapper is a very specific thing. It means "Neat and trim" but also has the connotation of put-together, graceful and somewhat elegant. This word also bears a definition of "lively and brisk" which adds a bit of energy and eagerness.
To look dapper one must iron one's clothes, be well-groomed and sport elements of classic menswear. Ties, suits, suspenders, fedoras, etc.
Dapper today holds strongly to the classics.
To be dapper:
+ think "classic menswear"
+ put a spring in your step
+ listen to Fred Astaire
+ keep your look tidy and polished
(image source: Modern Hepburn)