Also depends where you're going and what you'll be doing on your trip.
If you're going to need hiking boots, I'd probably wear those. Put your snow boots in the checked luggage.
If you want to stand out looking like an American, wear a graphic t-shirt with some regional logo, or sports team and Nike shoes. A baseball cap tops off the look perfectly.
If you want to blend in, dress as bland and non-descript as possible with minimal branding visible. As mentioned, dark colors, long sleeve, short sleeve as appropriate.
If you're changing climates/seasons, you'll want to layer. They will have blankets available on the aircraft but it's good to bring your own light jacket onboard.
I've got a Marmot jacket that I wear the inner liner as a "puffer" jacket by itself, or wear the outer shell by itself if it's raining. If I'm going to Boston in January, I'll zip them together and wear the whole jacket. For most situations, just the inner liner by itself is good and if I don't want to wear it, I just attach it to the out side of my backpack.
Personally, I wear dark blue jeans always. Rare exceptions, I'll wear Dockers or something like that. Back to my American comment, I spent a lot of time in Australia, SE Asia, Japan, etc. Nobody wears khakis in those markets. I don't think they wear them in Europe either. If you want them to think you're French, wear something ridiculous looking like red pants.
For me personally, traveling to Australia, I felt comfortable in a graphic t-shirt. Traveling to Asia or Europe, I always wear a collared shirt. And try really hard to make that NOT a work related logo shirt. If you want to wear a golf/polo style shirt from your favorite F1 Team, Man U, Bayern, Real Madrid, that's fine but super-mega-computer corp, definitely not (I mention this because corporate logo shirts are half of my wardrobe).
As mentioned, if you walk in like you are somebody, you'll get treated like you are somebody. If you walk in looking like you just came from the gym, you're probably not going to get those little extras - whether that's a better hotel room, a free drink, help in the taxi line, etc.
On the extreme side, I've got a friend who wears the same jeans for 2 weeks and washes out his backpacking underwear in the hotel room sink every night but... that's not for me. I check luggage like a normal person.
