I worked with the folks at Centerline Consulting; they were great. Their prep was equally helpful for my interviews at UAL, SWA, AA, and AS. I don't necessarily think you need to do specific prep for UA beyond that, although aviation interviews.com was a great resource too.

The technical portion of the UA interview is really easy if you're a current turbine pilot, which I'm assuming you are. Mine was literally just a couple questions on descent rates needed to make some STAR restrictions. There's no simulator or CRM scenario.

Good luck!!