I've been a business consultant for a software vendor for just over two years now. Turns out there are some things worth knowing..
Remember people's names. Not just those you deal with directly - that's a given. But also those you come across when you're workshopping or conducting training. It gives them a pleasant surprise to realise you paid enough attention to know who they are.
People will always want to fix the problem. They'll probably already have a solution in their head. Don't let this distract you from defining the problem properly first. Sometimes the thing that's causing the problem won't be fixed by their proposed solution.
Provide good advice, but understand that they might not take it. Be ready to walk away from what you think would be best. If you can't do this, don't be a consultant.
Have fun and work with good people. You don't have to be best friends, but why waste time and energy surrounding yourself with people who don't click?
I'm sure I'll think of more - any additions?