Hey Graham,I haven't had much luck with VAs/outsourcers over the years. I spent more time writing details, etc, only to have them not perform than it would have taken to do the jobs myself.
I finally figured out that to get results, I had to pay more than the other guy.
Let's assume you have a smart VA. He/she is probably working for someone else at the same time. Who are you going to give priority to? The boss that pays the most!
You need to offer bonuses for work that's done on time, etc. Offering bonuses is the ONLY way I ever got results.
Having said this, you should get a VA to do the things you either can't do yourself, or you dislike doing yourself the most. In my case, that would be writing.
I would want someone to write, write, write...Chunks, articles, tweets, etc. Writing good, solid content is what holds me back more than anything else.
If you get into things like linking and blog commenting, you are dependent on your VA's common sense and ethics. You need to make sure that these values are the same as your own.