G-B, Items I think you should have ready are:
seat bag
tire tools
new tube
patch kit
mini pump
water bottles
mini multi tool

If you're transporting to a ride:
air pressure the night before and again the morning of the ride
take your floor pump if you're in a group, someone's gonna need air
Lube the chain the night before, important if transporting inside of a vehicle

At the start of the ride:
Seat bag and mini pump on the bike
Quick releases are tightened
Quick brake opening lever closed (what's the proper name for that lever???)

After the ride:
Check for loose spokes
If caught in the rain wash and relube

I'm sure there are items left out that are useful, but I tried to only list what I feel are essential.

Brad