What's your location? This makes a big difference.

General points.
Do you need planning/zoning permision? If so, do not even think about doing anything until you have it.

Get references from people you know locally who've had work done.

Find out what the contractors plan to do themselves, and what they'll sub-contract. Specialists - plumbers, electricians etc-not being available is one of the biggest causes of delays.

Find out if you can help- can you clear the site etc? This can save days of time for the builders, saving you lots of money.

Get lots of quotes.

Get a plan, and stick to it. You start changing things half way through, you'll pay several times over in time, money and bother.