The purpose of a complaint letter is to bring your grievance to the notice of the concerned party so that corrective action can be taken wherever possible. While writing a complaint letter, it is important to address it to the right department. If the letter does not reach the right people then the purpose is defeated. Similarly, you must include your contact details as well.
Complaint letters need not be strong worded and rude. It is more effective to put forth your complaint in a firm yet polite tone. It is also a good idea to suggest what kind of remedial action you would like the concerned party to take.
State your complaint in a simple and concise manner backing up your case with photocopies of receipts or other relevant documents. Stick to the facts and avoid sweeping statements deriding everything in general. Keep the letters to the point and short.
Tact can achieve what sarcasm and rudeness cannot. So, try and include a couple of positive points as well along with your complaint. This is sure to act as a balm to the bruised ego and your complaint would be taken in the right spirit.