You need to include your research interests -- be as specific as possible. How will attending this specific institution allow you to pursue those interests? Why are you choosing their program? Do not include cliche fluff.
You need to open with this and integrate your past achievements/experience as supporting information for what you're planning to do.
Also avoid general and meaningless words and fluff (ex. I gained valuable knowledge -- this could mean anything and everything). You need to be specific. Do not include stuff like "this will help me in my career" -- they know this. This will also help many other people in their career -- why should they choose you over them?
Do not pad with irrelevant information or information they'll get from other parts of the application (ex. they'll know that you got good grades because they'll see your transcripts).
- For Teachers