DO involve your architect or designer early on - before you start demolishing, buying materials, building or landscaping. It's much easier to be flexible with ideas and designs if you have not already committed yourself emotionally or financially to doing things a certain way.
DO live in a home you plan to renovate or spend time in a place you plan to build before making too many decisions. This will give you an idea of sunlight, wind, views, privacy, use of space, traffic flow and other issues you may need to consider.
DO your homework. Collect images of homes and interiors that appeal to you - magazine clippings, books, movies, websites, your own photos and sketches - and use these as a starting point for discussing concepts and details with your architect or designer.
DO consider your budget and make sure it is realistic before you start building. It's far better to work in stages or modify your plans slightly than to end up with a second-rate job because you had to cut corners, or worse, live in a half-finished home for months or years.
DO be clear about what you want and work closely with your architect or designer to achieve this. And if others try to convince you to change the plans or materials because it's a little cheaper or easier, talk to your architect or designer before accepting their word. By sticking to the design and discussing any changes, you will end up with a cohesive, quality result.