in conclusion

Make sure that you like your traveling partners, it makes a big difference.
Buy tickets ahead of time if you can on the web, you will save tons of time.
Try different things, food and places, you never know what will be a pleasant surprise.
Talk to the locals, they know a lot more than you do about the visiting city.
When in doubt... just do it.
Get the best guides you can afford, it makes the viewing experience a lot more interesting.
What they call a martini in France, is NOT what we think a martini is. (we have first hand experience on that one).
Dress appropriately, it sucks to not be able to go into a site that has a dress code.
Try to leave your cell phone home, it is a pain in the ass when someone calls and it is 3:00am (is that correct Eileen?)
Try to pack light... (that's a lesson for me)
The house wine in France is better than most wines you will drink in the US
Sleep when you get home, there are things to see out there.