If you're ready to make the switch from a child-restraint seat to a booster seat -- because your child is at least 40 pounds and 4 years old -- you're probably wondering which kind to buy. Up until recently, there was little information available on how well various seats protected kids. But recently the Insurance Institute for Highway Safety tested 41 models to determine whether they properly positioned the lap and shoulder belts on young children. Ten of the seats made the best-bet list, 5 were good bets, and 13 weren't recommended. You can start by checking the best bets, but because models are often updated or discontinued, use the tips below to help you find the best booster for your child.

Don't rush the switch. If your 4-year-old hasn't hit the weight limit, keep him in a child-restraint seat, suggests Ann McCartt, Ph.D., senior vice president for research at the IIHS. "More and more restraint seats have higher weight limits, some as much as eighty pounds, so you can keep your child in one longer. And these are really safe." However, if your 3-year-old has already reached the weight limit of his car seat (many max out at 40), you'll have to make the switch sooner.

Decide between a backless or a high-back booster. High-backs tend to position the shoulder belt better than backless because they have shoulder-belt routing guides. They're also a good choice if your car doesn't have a headrest in the backseat. Backless boosters tend to be cheaper, but be sure to look for one with a clip that can help ensure the shoulder belt stays in the right spot. These boosters also have better lap-belt placement. "It's possible to find both types that do a really good job," McCartt says. (Combination seats that convert from a restraint seat to a booster are also available, but these types didn't fare as well in the IIHS tests.)

Check the fit. Once you buy a seat, test it out. Some stores will let you test floor models, which McCartt says is a critical pre-purchase step. The main goal of a booster seat is to raise your child up so the adult seat belt fits well and protects your child in the event of a crash. "The lap belt should fall across the upper thigh—not the abdomen," McCartt explains. That way, the force from a crash is absorbed across the bones of the pelvis, not the soft tissues of the stomach, which are vulnerable to internal injury. "The shoulder belt should fit snugly across the middle of the shoulder, not too close to the neck but not falling off the shoulder either." Despite these guidelines, as many as 65 percent of kids are still not properly restrained, according to a study conducted by the Automotive Safety Program at the Riley Hospital for Children and Indiana University. Researchers found that kids' shoulder belts were often slack, or even tucked behind the arm or back.

Spend Wisely. Remember, pricier seats don't necessarily offer more protection. Some of the seats that made the organization's best-bets list were some of the cheapest, says McCartt. "For instance, the Graco Backless TurboBooster sells online and in stores for about twenty dollars."

Don't stress out! If you already have a booster seat that's not on this list or is one of the seats that's not recommended, you don't have to go out and buy a new one until you test it. "Our ratings captured the likelihood that a booster would work well in all types of vehicles, but you need to see how well the seat works for your child and in your car," says McCartt. Plus, what's most important is that your child is in one, period.

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