Waiting fees?
Tell the clients that if you have to wait more than 15 minutes, you charge a waiting fee which is $10 per 15 minutes. Make them pay. Make them respect your time, or they won't. Some people will keep you waiting all day without batting an eyelash. Movie sets, directors, and attorneys will often make you wait. Since many people find waiting fees distasteful, you might charge a high travel fee for folks that you think will keep you waiting, and then patiently wait without complaining.