I've honed this pretty well (IMO) after a few long overseas trips:

Pockets (including blazer):

• Passport
• Wallet
• Small notebook with pen
• Pocket-sized phrasebook (if foreign country)
• Phone
• Facial tissues
• Front breast pocket for lens cap, if carrying SLR
• Metro map, if I don't know the city and am using it a lot
• Boarding pass, rail pass, etc.

Satchel:

• iPad
• Map, if needed
• Guidebook, if needed
• SLR, if needed (plus memory cards + extra batteries)
• Kindle
• Magazine (Economist or Foreign Affairs, generally)
• Pen, pencil, notebook
• Anti-bacterial lotion
• Earbuds
• Booklight
• LED flashlight
• Emergency cash ($200 in crisp twenties)
• Chapstick
• Laptop, if needed
• Scarf and gloves, if needed for the climate
• Flash drive of important documents

If it's a business trip I'll probably use a different bag. I have a leather satchel I use for personal trips and a Samsonsite laptop bag for business. The Samsonite bag is much larger though still fits under the seat in front of me. It's got room for documents and a leather folio.