Not my tips on this occasion, but a recommendation for a website post on active/passive voice. It can be found via this link.
This is one of the things I have to put my hand up and say guilty. My writing from ten years ago was all passive. These days I've got my head round it and try to avoid it, but bad habits die hard so I still find myself slipping into it every now and then. The above link is a good summary of what it is and what you need to look out for.