By far the most important tip, is to style yourself in a way that is authentically you, and that you love and feel most comfortable!
1. Fabrics, textures & colors. Mix things up a little and add some depth by wearing different fabrics and textures, denim, wool knits, silk, linen etc. In a family shoot, co-ordinate outfits using a complimentary color palette, but avoid being too matchy matchy!
Stay away from graphics and florescent colors. Please, don't let your kids wear those athletic shirts that are bright neon orange or yellow. What that means is that your kids face will be bright neon orange or yellow. Graphics tend to go out of style so I suggest staying away from them.
2. Be yourself! Most important tip. Be yourself!! If you don't usually wear dressy clothing, don't. If your daughter hates dresses, don't put her in a dress. If your son can't stand wearing button down shirts, don't put him in one. You can find clothing that fits your style that is clean, nice, and comfortable. That doesn't mean that you shouldn't dress your best, but it does mean that you can dress your best in your own style.
3. Consider the weather. Dress in layers. Layers add interest but also allow for removing clothing if it gets warm. Think scarfs, hats, vests, cardigans, jackets....
4. Know your best features. Legs? Wear a shorter skirt/dress or one with a split. Arms? Go with something short sleeved or sleeveless. Love your waist? Try something belted or fitted.