First of all, a good valedictory speech must be sincere and should echo the speaker's heart and mind. It should be emphatic and should use words that are fresh and alive. Basically, good valedictory speeches should contain three important parts: an expression of gratitude to the right people, a brief lookback of significant events in the past, and a motivation for the graduates to pursue their goals and ambitions in life. They may not necessarily come in that order, it is up to the writer to work his way around the three parts. But, here's the but, never ever repeat someone else's speech. Try as much as possible to avoid cliches which are usually rampant in speeches like this. So, just be yourself and deliver your speech effectively by seeing to it that the three important parts are there.