Ask HN: What design services have you had luck with?

A fellow developer and I are about to start working on a new Web App in our free-time, and I wanted to use a design service to expedite the process.

If you have used a design service before: what was the service and would you recommend it? Any tips or things I could personally do to get more out of the service? All feedback is appreciated!

I have been lurking on this site for around two years now and this is my first post, hooray! :D