When you're leaving a job that was a bad experience, especially if it was due to poor management or working conditions, it can be difficult to maintain a professional attitude. However, even on the way out, it's best to take the high road when writing your resignation letter.
Keep the letter as brief and simple as possible. The main reason for writing it is to give your formal notice. The most important information to include is the date of your notice and the date when you are leaving. This should be within the reasonable, agreed up period of notice for your position. If applicable, you should also offer to help with the transition or train your replacement.
There's no need to go into detail about why you are leaving. This is not the place to air complaints or grievances about your manager, colleagues or working conditions. It is also in poor taste to brag about higher pay or benefits offered to you in a new position. You can either not give any reason or say that you're “moving on to other opportunities.”
Try to focus on the positives of the job you're leaving and thank your employer for what you've learned in your time there and for opportunities given. If it was a stressful job, perhaps you've learned to better manage stress or get along with difficult people. If the job wasn't a good fit, at least you've learned what doesn't work for you. There's something to be grateful for in any situation.
Remember not to burn bridges as you may need a recommendation from your soon-to-be-former employer. Keep your relationship cordial and leave with a good impression.
Keep it brief and simple
Avoid giving specific reasons/examples
Focus on the positives
Don't burn bridges