For a while I was sitting on a yoga ball. Reduces the leaning forward, but obviously you have no arm support. But it reminds you to keep your back straight.
I have only sat in an Aeron chair once or twice, but that supposedly makes a difference.
There is another type of highly adjustable task chair which costs about 400 clams, trying to remember the name of it, but it is a good one.
As for dress – I wear kakhis, jeans or cargo pants and a sportshirt or polo type shirt 99% of the time. The suits come out for trade shows or shoots at conferences. We used to wear ties when clients came to the office, but now business casual is ok. Visits to a client is sport coat and tie, unless it is Friday, you can often lose the tie.
The best attire is for shoots in the operating room – those scrubs are like wearing pajamas – unless you get the tyvek jump suit – pure sweat.
I agree with the others, unless you work in a strict corporate setting where everyone wears a suit, or ad agency suit=loot, wear what you are comfy in.
Or do what I do – wear what the boss wears.