Parenting magazine recently reviewed the Insurance Institute for Highway Safety "best bets" for booster seats. I will include the link (below) and summarize some safety highlights for use when you are shopping for booster seats.
Most children are ready to switch from a 5-point restraint car seat to a booster seat at about 4 years old and once they have reached 40 lbs. All children are different, so some three year olds may be ready to switch if they have reached the weight limit of your current car seat. However, many restraint seats now have higher weight limits. If your child is still comfortable in one, there is no hard and fast rule saying that you need to switch.
You will need to decide between a high-back booster seat or a backless one. High-back boosters have shoulder belt routing guides, so they can position the shoulder belt more appropriately for your child. High-back models should also be your first choice if your backseat doesn't have a headrest. Backless boosters tend to be less expensive, and may have better lap belt placement.
If you shop in a big box store, most of the time you can try out a floor model. This is important, as seats can fit different children differently (for example, some seats are more narrow, and may be uncomfortable for your child).
The main goal of a booster seat is to elevate your child so that the adult seat belt fits well and protects your child in case of a car crash. The lap belt should fall across the upper thigh, not the abdomen. That way, the impact of a crash is absorbed across the pelvis and not across the soft tissue of the abdomen, which is more vulnerable to injury. The shoulder belt should fit snugly across the middle of the shoulder, not too close to the neck, but not lax enough to fall off the shoulder either.
Always check your child and the placement of shoulder and lap belts each time they get in the car, as the seats themselves can slide around in the backseat, away from the ideal location.
http://www.parenting.com/blogs/show-and-tell/more-booster-seats-earn-top-safety-ratings
