Here are some rules which should make your life easier as an IT consultant!

1) Test, test, test

After setting a system up, test the crap out of it. Can the user log in? Is their email working? Can they send and receive externally and internally? Can we restore from backups? Have I accidentally left part of it unsecured? Can I get in remotely to fix stuff if it goes wrong?

2) Reduce a problem to its simplest possible components

If you don't understand part of a problem, see if you can take it out. For example, if someone's using a mail server you haven't seen before, see if you can change the MX records to move to a server you do understand.

3) Never use a new product for the first time with a client's system

Always try to play around with something before you use it for a client. Make sure that if it breaks, you're the only person to be inconvenienced. Make sure you understand something before trying to make money from it.

4) Under promise and over deliver

Don't tell a client it will take three days if you think it will take three. Tell them five, or a week, they probably won't care. Don't put yourself under unnecessary pressure!