For the guys: Work on kick serves and actually practice returning slow short serves.
The most effective guy is the quickest at the net who can cover the most court. In mixed, I'm not that great a partner since my movement at the net is relatively poor and the match can be dominated by the opposing net guy if he is exceptionally quick.
When I played 8.0 mixed with my first ex-wife who was an excellent volleyer - she learned to volley first when learning tennis - she played the center of possible returns at the net and I played back and covered both alleys. I was very fast back then and we played to our individual strengths.
We only ever lost one match - to the national championship team - and I had an easy volley on a match point for the win I dumped into the net.