Great forum as I recently hired two subcontractors!
This is how I've worked it out:
1) A CONTRACT including a NDA and non-compete clause. (Jean Mejia of Your Virtual Office Assistant (virtualpensacola@gmail.com) has a great contract one, my lawyer had me change a few things but mine is basically like hers.)
2) You need a system to track documents, who's doing what project and all the details of each project; we use Google Docs. (I have a Projects & Task page that lists each project, the client, their contact details, when project assigned, when projects or milestones are due, and a comments column. I use different colors so I can see at a glance who has what project.)
3) You need to discuss details that may or may not be in your contract. I think these should be in the contract but lawyer says not, it's in a addendum: Hourly/project rate, exact duties, learning curve pay, pay dates, etc. (I do NOT pay for learning time as it's a skill the sub will take with them when they leave. Notice I said 'when' not 'if'. Subcontractors will eventually leave your practice when they decide they've learned enough from you or they want to make more money, or the two of you have 'creative differences'.)
Both of my subs have a minimum hourly rate (which is what they offered when they contacted me) and I bill the client my regular rate, Both of of us make money and more importantly; my clients are happy and our reputation is growing...
I built a Google spreadsheet that lists each person's skills, software they own, online programs they know, and other skills they have such as typing WPM, or 10-key expertise. I also have a time sheet for each sub that shows the days & dates of the week worked, the client's name, project, billable hours, total hours, and date paid. I pay through Pay Pal and their fees are built into my rates as I consider that part of 'doing business'. I know some VA's charge for everything from stamps to envelops, but I can write those items off my taxes.
I hope this helps!
Viki