Perhaps make a point about setting specific objectives and review those objectives on a regular basis giving praise or guidance. Give your assistant a vision of how you believe the office should be operating and discuss with her why you believe her input might be falling short of that vision. Try to understand her worries and concerns and where possible act on them, but where not possible (i.e. where the behavior is totally unacceptable) make sure you say so. Communication is the key. You might find you will gain, over time, some respect.