Treat your art like a business
When you begin accepting money for your art, it's no longer a hobby. You've taken your first step into the professional world. You don't have to be living off of your commission income to consider it a business, but when money is involved, you need to treat the transaction with a higher standard than you would gift art or trades. Failure to do so can scar your reputation for any future business. If you're not prepared to take on the responsibilities, stick to gift art.
Price your art as if you were working hourly
It's okay to offer dirt cheap commissions when you first 'break into the business,' but as you start to get customers, you need to consider the value of your art. If it takes you 1 hour to do a sketch, and you're offering $3 sketches, you are making $3/hour. That's less than half of minimum wage. Price yourself fairly, but if you find that the value of your art exceeds what people are willing to pay, compromise by offering a