First and foremost is the cost of the house. You need to have real idea on the budget, including cost of property, taxes/fees, permits and any other thing that will take from your bottom dollar. Discuss with your bank the going cost/sq. ft. in your area so that you know if that 2,500 sq. ft. house is affordable. Nothing worse than getting to a point and realizing your "eyes are bigger than your plate", causing design rework and construction stoppage.
Something the whole family can participate in. You can take pictures of houses that you see driving around town or on vacation, magazine pages or website pictures, another drawing or even your own sketch. A picture is worth a thousand words. And when I have an idea of what every family member enjoys and needs, my job is easier. Start this scrap book months in advance.
Start a list of what you like and don't like in your current residence. No house is perfect after we've been in them a while, but it was almost perfect when we first walked through. Now that your home has grown on you, what do you really like and what don't you like. Do you wish the laundry room was bigger, on the second level? Did you like the idea of the walk in pantry, but not the size? Or, since the kids are growing up or out on their own, will three full baths really be needed?
Know the kind of style you like inside and out. I can usually mesh a style or two, so don't worry if two don't totally agree.
You know the house you have imagined in your head, you've done the research, have the sketches and wish list. Now the fun part.. shopping, well kind a.. Start getting quotes and collecting brochures, manufacture catalogs, flyers for items like: appliances, light fixtures, flooring types and colors, paint color swatches, etc. This will help me achieve a more perfect design to match your vision, BUT also will get you to start thinking cost. (see "first things first" at the top of this page)
Have you ever watched HGTV or any other home show. They list everything they want, but rarely get it all. If money is no object, then forget this part, but if it is, what are the most important items on your list to you? The size of the kitchen if it means the mud room might have to be reduced in size or deleted? More than once, I have had customers come back to reduce a room or size of overall house once they've talked to a banker or builder. I expect this... I expect changes... Its all part of the process.