Register using your Facebook account.
This increases security as it allows users to do background checks on each other. Knowing that you have a mutual friend really helps.
Set your profile properly.
Use your real name, a picture and license plate number so that other users can find you as well trust you.
Don't take too long to reply.
Be courteous and reply when a passenger requests a ride or chats with you. You can come to an agreement on the fare, time and final pickup/drop-off locations too.
Being late is often disrespectful and bothersome to fellow carpoolers. If you are running behind schedule, simply just let your passenger know so they aren't waiting unknowingly.
Don't forget to rate your ride.
More ratings create a more credible community. This will also help increase your chance of being requested for a ride by other passengers.
Build a community.
Build a network of passengers. Don't just rely on one regular carpooler to compensate for sudden unavailabilities.