It sounds simple, but telling them you trust them to make a better product than your mockup helps a lot.

Give them creative freedom to solve your idea in their own way. You may understand the technical specifics and big picture of what you're trying to do, but acknowledging that they know their craft better than you do (that's why you're talking to them) and that you want them to design it out in the way they think is best - just saying that will go a long way. Giving them a mockup helps - but let them know you're ok with their interpretation veering from that.

Obviously if the person you're talking to is an awful designer, nothing will fix it; but if you have a good one - the ideas that will flow will make your product awesome (even if they aren't implemented in the end).