Classic clothing, like khaki or denim, is timeless. Coordinating solid color tops with jeans or khakis works well. The eye is drawn to bright areas in the photograph, so no one should wear white unless everyone is wearing white.
Patterned outfits (plaids, stripes, and prints) are generally avoided, because they can distract from the portrait. You want the viewer to be looking at you- not the shirt! Patterned shirts, dresses, or skirts can help add a little flavor to a picture though, if done right. Think of what color background you have decided to go with for your picture, and how your outfits will look against the background color.
If you are having problems with picking out outfits, an easy way to help visualize what you want is to Google “family portrait” images. Now, you have thousands of pictures at your disposal. Look for pictures you like, and see what kinds of outfits different families chose for their portrait. You may find yourself consistently liking and not liking certain things in the pictures. You might really like matching family outfits, or dislike when people wear thick stripes. This is a great way of helping yourself decide what you want to wear. If you want to see even more pictures, use another search engine as well such as bing or yahoo.