Tried and tested on long flights (10 hours on the Dawn to Dusk):
Comfortable trousers that unzip to become shorts; nice if weather is likely to change.
T-shirt in a dark colour - impossible to do an A check properly, re-fuel etc, and still keep a white one even relatively clean.
A jacket you can take on and off easily; it can be warm in the air and cold and windy on the ground.
Above all clothes you feel comfortable in, in all ways. If image is important to you, you won't be able to convince yourself it isn't, so find a way to be physically comfortable and look how you want to look. If you really don't care, that's fine, and makes it a whole lot easier.
And if you fly an R22, you can't take anything on or off one-handed, so you have to get used to being too hot or too cold, and just ignore it.
