Don't go for too much on the first volley. Just hit every one to the opposite corner. Opposite corner meaning if you just served to the deuce side, hit the first volley to the ad side. Serve to ad side, hit first volley to deuce side. Simple. Don't go for too much. Just get it nice and deep.
I really think this is the key for most people, playing the first volley in a high percentage fashion, deep and safe into the corner. When I see people who suck at serve and volley, most of the time it seems like they go for broke on their first volley.
After the first volley, close in tight, cheating towards covering the down the line passing shot.
Recently, I've been noticing that my serve and volley is actually pretty decent. It's not because I've been practicing it, or my volleys, that much, it's just that as I've gotten older I go for less. My volleys are much more compact, I go for much less, and every so often I discover even the shortest stroke, when it makes perfect contact with the return, is enough to put the first shot away. A lot of tennis is advancing to the point of realizing, and embracing!, how simple the game is.