It is important to clearly identify the problem. You need to explain why you are dissatisfied and what you would like done to correct the problem.
Make sure you have proof of payment or proof of dealings with the business. Make copies of receipts, canceled cheques, guarantees and any other correspondence between yourself and the business. Never give away the original copy.
The best approach is to be polite but firm. Do not lose your temper or your sense of humor. Be persistent. Do not give up until you feel you have received a satisfactory response.
It is important that you be reasonable and fair. You should determine the business's refund policy before making any purchases and ensure that you have properly complied with the terms and warranties. You have no legal right for a refund unless the goods are defective.