I've played a lot of 8.0 mixed doubles, and a lot of 9.0 mixed doubles too. I'm a strong 4.5.
In mixed doubles, my favorite strategy is what others have described as the L formation. I park my partner extremely tight on the net. Her job is to defend the balls that come right at her. The key is to make sure that she never backs up. Lobs over her head are my responsibility -- once you get used to doing it, you'll realize that rolling diagonally backward left to right behind your partner to hit an overhead on a lob over your partner's side (when you are righthanded and playing the ad court) is actually easier than running directly backward to play an overhead on your own side.
The other key strategy: I always serve from Aussie formation in the deuce court on my serve. The reason for this is that the standard doubles formation invites the opponent to hit a lob return over my partner (forcing me to play either a backhand overhead or a high backhand volley, which is difficult). The Aussie formation effectively takes away the lob return because the opponent has to fear my overhead when they lob over my deuce court.
In my experience, matches in 8.0 mixed doubles are generally won by the team with the strongest guy on the court, since a strong 4.5 can usually dominate against 2 4.0s. But in 9.0, the opposite is true, and the team with the strongest gal usually wins.
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