Less Is More
Fine prints (think pinstripes or small scale florals) and understated solids are a safer bet than, say, a bold geometric pattern or neon orange.
Keep It Classic
You want a timeless look that won't seem out of place when your child's looking back through old school photos years from now. Think pretty dresses with clean lines for girls and tailored collared shirts and jackets for boys.
Collars Are Good
A reasonably sized collar is always polished and also a great way to frame your child's face. Try a simple Peter Pan collar for girl or a classic polo or button-up for boys.
Accessorize With Caution
Headbands, barrettes, bows, suspenders, all can be used in moderation, but try not to pile them on and consider scale. (A bow the size of your child's head is probably a bad idea.) Also, it's important to note that you cannot control where the accessory will end up (well-meaning teachers and photo assistants have a tendency to make last minute style tweaks), so if a center top bow is not the look you're going for, consider ditching it altogether.