I'm with you on this. I hate it. Business trips are a packing pain in the ass because you need 3 sets of clothes (business, schmoozing and casual) AND enough variety to present a different enough outfit at each business/schmooze setting.
The key is picking highly versatile background items (suits, shirts, trousers, etc) that can double-up without people noticing, take enough accessories to add the variety (ties, pocket squares, shoes) and remember that hotel laundry is your friend when it comes to shirts, underwear, etc (but NEVER give them anything really expensive).
For a 6 day business trip (assuming business meetings requiring suits during the day, a handful of more casual business schmoozy events in the evenings, and a few more casual nights out), a minimal rotation would include:
2 suits - versatile dark solids or subtle stripe
2 pairs of dressy trousers - any of dark grey, navy, black, olive, light grey or tan (pick one light, one dark).
3 shirts - white, sky blue, pale pink. Pick solids with some textural interest rather than memorable bold stripes. Hotel laundry will let you last 6 days.
2 pairs of shoes - black oxfords & brown monkstraps (but see below)
3 sets of underwear & socks - hotel laundry will let you last 6 days.
6 ties - if you need to wear a suit every day, you'll need them. Conservative & versatile patterns - rep stripes, pindots, etc. Maybe one less conservative & memorable one that's still tasteful one.
3 or 4 pocket squares - one white linen, the other two or three chosen to complement any of the jacket/shirt/tie combos you're packing.
1 sweatervest - navy
2 belts or 2 pairs of braces depending on what suits/trousers you have.
toiletries (obviously, don't take anything the hotel provides, and if you're really stuck for space and don't mind the cost, don't take anything that can easily/quickly be bought locally on arrival instead).
pyjamas
laptop
phone charger (for 6 days, you'll need it, if you make enough calls. Shorter trips you don't need it)
any work-related junk
phone, wallet & keys & nice rollerball pen (leave behind any unnecessary cards/keys/etc)
camera if you're a shutter-bug (charge it up before travelling, so you don't need the adapter)
passport & tickets
You should be able to fit all that into one folding-suit-carrier type luggage, and one carry-on bag. Job done.
For travelling there and back, you can wear jeans, smart loafers, a versatile sportcoat, plus a oxford button-down collar cotton or chambray shirt, and anything else needed for the weather when you get to Chicago (for 6 days, you can get away with a single overcoat/hat/gloves/scarf combo, though it's painful). The jeans, loafers and sportcoat give you extra options to mix/match with the other items in your luggage when you get there, so you actually have 3 pairs of shoes in total (enough for 6 days of business wear if you're careful with the choices).
If you have room left over - add more shirts/underwear/socks to avoid laundry costs and if you still have more room after that (possible), add in a pair of dark brown oxford shoes or another suit.
