Due to 50 years of the press showing works of 'art' being made by elephants with brushes held in their trunks, monkeys, guys riding over canvas with bikes, kindergarten children and anyone else with no talent and no training; it is increasingly difficult for any member of the public to believe a professional painter should receive a per hour renumeration that might be similar to what a plumber or an electician might recieve should they spend an equivalent time at a contracted task or job.
This has become the sad lot of painters so the point must be made forcefully and prima facie before any work is undertaken that you expect to be paid on a scale commensurate to your study, skill and experience; at least at a tradesman's hourly rates.
Here I will discuss just a few basic rules for graduated painters who intend to make a career out of commissions.

• Be professional. Make the client aware that your hourly rate should be adequate. 
• Define the work: Make sure the size of the canvas, paint, the mounting and frame are all costed and defined. In a lot of instances it is appropriate for the client to agree to pay separately for the frame. This does not preclude a caveat by the artist on the type of frame to be used. I find it useful to make this provision at this stage. 
• Take a deposit - at least 10%. More if you are including the frame and mention the cost of artist quality paints, particularly if you intend using any seriously expensive colors such a cobalt blue etc. 
• Settle on a completion date with + or - variations for unforeseen circumstance. It it is a portrait this time should run concurrently with the sitting times and the sitters availability. 
• Agree on the scene/portrait - general colors and style and if a portrait the mood and props. Don't rush this as it is most important. You should value a happy customer for it is from them you will get a plethora of new clients. 
• Be careful the client understands that you will exercise your skills to the fullest but not all paintings turn out to the clients perfect satisfaction. In fact the client should be made aware there are certain risks involved that are separate to actual performance. 
• If possible get your signature and your clients on a piece of paper! 
• And never, ever, take on more commissions than you can handle. Portraits particularly, can become very demanding whereas commissioned landscapes are usually a joy - particularly in summer. 
• Be professional.