Good advice above. My additions:
1) You should be able to nail multiple serve targets/placements on demand. Preferably with both a fast serve and a kick or slice serve. Kicks that are hard to return work well for S&V.
2) Day of match - figure out what is/not working primarily for you, but secondarily for your opponent. Serve placements, returns, different volleys (high, low, fh side better than bh side, all good, or bh weak?). Significantly - identify your opponent's strike zone on serve returns (where they hit good returns from). Avoid their strike zone.
3) If your opponent is good at hitting returns that are hard to volley - have a plan B. If they're nailing returns of your first serve down the line on both wings - time to consider a different game plan. I like to use S&V when it works to my advantage - not to go down in flames in 40 mins ;-)
4) Like others have said - mix up serve placement and keep your opponent wondering. Maybe 10% of time mix up something like a Kick instead of a bomb on a 1st serve.
I don't always serve to their weak wing; I might instead serve it right at them 30 to 40% of the time and up to 20% of the time to their strong wing. Don't always serve wide - even though it might give you an easier 1st volley. After a while it becomes predictable, and a good opponent will adapt.
5) Once you have a good idea of viable serve targets for this opponent/match - the patterns are pretty easy as spelled out above. Mix up hitting volleys away from the opponent with occasionally hitting behind them.
6) A subtle point - improve your overheads too, cause if you're really good at S & V - they're going to start lobbing you.
Good Luck! K_I