Hopefully mostly obvious points but a useful primer nonetheless.
- Be specific
- Don't tell the user where to click and what to do
- Use the user's language and not the company's language
- Have a correct solution
- Don't make the tasks dependent (if possible)
- Provide context but keep the scenario short
- Task scenarios differ for moderated and unmoderated testing
So, all very sound advice. If you're planning user testing, I recommend you read this in tandem with David Travis' six approaches to setting tasks.
Motivating usability test participants (previous blog post from December 2010)