"Do your best." Have you said this? Or what about "Just do your best." Is there a difference between those two phrases?

It depends on how they are said. I've often used the latter when employees say that they aren't capable of doing the task at hand. It's usually said with a sigh, "Just do your best," as if I'm already acknowledging that their performances will be substandard.

The first phrase, though, "do your best," can, if said in the proper chipper manner, indicate that this person should honestly do his or her best on this project. No stone left unturned, no detail ignored. There are very few times that this level of perfection is what you want. Most of the work your employees need to do is average work. 

If you want to motivate people to do the right kind of work, here are ten phrases you should use instead.

  1. I know you'll do a great job.
  2. Let me know what resources you need to accomplish this.
  3. We have a strict deadline for X. It will be impossible to do this perfectly in this amount of time. I trust your judgment on which corners to cut.
  4. Let me know what help you need to get this project done. I'm happy to help.
  5. I know you're concerned that you lack the skills to do this, but I know you can figure it out. I'm here as support.
  6. This project is critical, and it needs your top attention. Make it your priority and let me know what you need to drop.
  7. This is new, and we're not quite sure how to accomplish it, but I know you have the knowledge, skills, and abilities to figure it out. 
  8. This isn't a huge priority. It does need to get done, but don't stress out over it.
  9. Give it your best shot, and we'll correct any errors later.
  10. I just need a rough draft/estimate/outline/whatever.

Hopefully, you'll notice a theme throughout these examples. The idea is to be supportive but also to set clear guidelines.

What can you offer in terms of help? If this is a new area for your employees, make it clear that you understand this is a stretch, but you have confidence in your employees. It's always important to explain that you will support your employees. Or, if you can't, say that. 

It's about setting and managing expectations. Make sure you differentiate between the top work you need and the more rough work. There is a time and place for both. 

What would you say instead of "do your best"?