I always dress "nicely" for flying - what would probably be considered "business casual" and that includes a blazer so I don't end up with a wrinkled jacket for dinner in a great restaurant when I get there. I've found the treatment I get on the ground (check-in, gate, lounge, etc.) is almost always better, and in the air is usually better. But - please note - my manner of dress is always accompanied by a friendly, smiling attitude. The people helping (not "serving") me are doing the best they can with a difficult and every day a bit more demanding public, and I try to make their day a little easier, too. It usually is returned.

Not always (or even frequently) with upgrades or other costly rewards, but almost always with a smile, a "thank you for noticing" or a "I wish all our passengers were like you." THAT makes MY day!