What do you need to do to prepare to work with an interior designer?
First, you are going to want to collect samples even if they aren't the final choice. The samples will help you articulate the look you want.
Pictures from magazines are a perfect start so you can show the designer the type of look that pleases you. Have several pictures ready.
If you would like, go to paint stores or home stores and select paint-color samples. Even if you select 20 or 30 of them, just pick colors you like. It will get the ball rolling.
You can also have pictures of rooms you do not like so the designer knows what to avoid. This isn't mandatory, but it can be helpful.
If you have pieces that you want to keep, be sure the designer knows you aren't going to part with them. The designer needs to know about such pieces early on so s/he can work with those pieces.
If there are pieces you aren't sure about keeping, be sure to get the designer involved in that decision, too. A good designer will never say "everything must go," but instead will work with what you intend to keep.
Hire the designer as soon as possible. If you are building from scratch or remodeling, get the designer involved in all the planning stages and all the meetings with the architect and the builder/contractor. This way everyone is on the same page and potential problems can be eliminated -- or, at least, minimized.
Be sure all who will be living in the house are present at the meetings.
Many times husbands and wives have different opinions on colors, styles and what to keep and what to throw away. The designer can help guide the process in a way agreeable to all.
Some designers bill for local travel time and others don't. Be sure you know what you will be asked to pay for.
This is particularly true if you are paying by the hour. If you are purchasing all the furnishings through the designer, travel time should not be charged as long as it is local.
Be ready to think outside the box and be ready to keep an open mind.
A designer might suggest something that you don't like, and that's OK. But be open to possibilities.