Hi Tiffany,
I've never hired a sub in the true sense of the word - I've paid VAs to work on projects with me or for me, but always paid their regular rate. In any case, I think the lessons-learned are very much the same.
I think I learned (and wish I had known or accepted earlier on) that it is OK to have expectations (and communicate those expectations) when working with another VA. There is a fine line between respecting another service provider's business model and communicating your needs as the 'client' - it was hard for me early on to communicate what I wanted, or if something didn't meet my expectations, because I was worried about offending, or similar.
One more thing I've always taken very seriously when working with other providers: Always pay the invoices on time if not immediately upon receipt - this is common sense, we all expect/want to be paid on time for the hard work we've contributed. I know how bad it feels to have to chase money, so I never, ever put another service provider through that.
I'll be interested to see this thread grow, looking forward to seeing responses from VAs who have more direct experience and can answer your question more thoroughly