Many booster seats have a minimum weight of 30-33 lbs. However, a booster seat should not be the next purchase when the child outgrows a convertible seat. A child is safest if they remain in a harnessed seat until they reach at least 40 lbs. A combination seat should be used after the child has grown too tall for a forward-facing convertible, but is not yet heavy enough (40 lbs) to move into a booster.
It is also important to take the temperament and maturity level of your child into consideration.  To sit in a booster seat, the child must be capable of sitting up without slouching or leaning over, keeping the belts in place, and staying buckled.  If your child is unable to do so (whether because of age, maturity or personality), a higher weight restraint should be considered as the next step.

I strongly recommend that you take your child with you when shopping for a booster seat. Sitting the child in the seat will give you an idea of how it fits now and will fit several years down the road, how comfortable it is, and whether your child likes it. Here are some features to look for when searching for a seat that will fit your child:

Belt fit A booster seat is designed to properly position the lap and shoulder belt on a child.  Try the seats out in your vehicle.  The lap belt should fit snug across the upper thighs and not ride up onto the tummy.  The shoulder belt should comfortably cross the child's shoulder and not fall off onto the arm or cross the neck.  Most boosters offer some type of "arm" that helps keep the lap belt low.  The shoulder belt guides should help keep the shoulder belt in position.

Length and width  The child's legs should hang comfortably at the edge of the booster seat without the child having to slouch.  The child's hips should fit comfortably between the sides without being squished or having too much "play".  Some booster seats offer an adjustable width (Britax StarRiser) to help accommodate growing children.  Be sure that the booster seat length and width fits the child comfortably now.  A shorter booster seat may need to be replaced later on if the knees are hanging off the end, making the thighs uncomfortable.

Padding A seat with minimal padding will make the child uncomfortable and cranky on longer trips. Some seats have the padding in the cover, others have padding underneath. Don't hesitate to take the seat apart and check out the underneath parts to be sure your child's body will be well-supported.

Seat height  If you choose to use a high-back booster, be sure you note how high the back is.  For the growing child, the higher the seat back, the longer they'll fit. The seat back is outgrown when the tops of the child's ears are even with the top of the seat.

High back or backless Which type of booster you get depends mostly on your vehicle.  If your vehicle has low seat backs or does not have any head restraints, you will need a high back booster to provide whiplash protection for your child.  If your vehicle does have head support, a backless booster is fine.  However, some younger children prefer a high back style, as the side "wings" allow for more sleep support.  Also, some older children prefer a backless style that does not "show" as much and cause teasing from peers.  You must be absolutely sure that your child has adequate head support before using a backless style.  Remember, safety before popularity.

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