There are a few threads -- if I remember correctly -- on this very topic over in TravelBuzz. Look, I found some for you:
Dressing Up When Traveling
http://www.flyertalk.com/travel/fttr...ML/001629.html
'Young' and Flying J/F
http://www.flyertalk.com/travel/fttr...ML/011069.html
Dress code
http://www.flyertalk.com/travel/fttr...ML/002252.html
As for myself, I always tend to dress "snappy casual" -- partly because of my nature, partly because I believe I do get better service and respect. That, combined with my wit and charm, has helped me numerous times. YMMV.
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"There are those who travel, and those who travel well."