My advice is to hire a good, stable person who have already proven her/himself to be a successful admin/exec assistant.
In terms of managing them, one of the best tools is a weekly review session. What were the goals/projects were supposed to have worked on? What happened/where are they with them? Goal/focus setting for the next 7-90 days. Any professional or personal problems, questions, concerns they want to share or ask about.
If you do those and are genuinely interested and listen you will build good communication channels, start nipping problems in bud, and get a lot more done.
Two resources for you:
There's a gentleman by the name of Barry Shamis who has a program entitled, "“How To Hire The Perfect Administrative Assistant”.
Also the book "Managing Up : How to Forge an Effective Relationship With Those Above You", by Roger Gittines, Rosanne Badowski. Rosanne worked for Jack Welch at GE.