Basically a pitcher's strategy can be broken down into two categories, you've got command of your pitches, or you don't.
It doesn't do a pitcher any good to approach hitters with the same game plan when his stuff isn't as sharp as on other days. This is where mental preparedness and toughness comes in.
When you've got command of most of your pitches, the biggest problem most pitchers face is a lack of concentration. We've all seen it. A pitcher is sailing along and all of sudden, he starts getting drilled. Quite often it isn't because he's lost velocity on his fastball, but more likely because he was having so much success, he lost his focus.
A pitcher can stay focused and sharp by having another pitcher or coach keep a pitching chart, and review it each inning to see what you are throwing and when. Compare that to who is coming up in the next inning and how you can approach those hitters. This keeps you focused and helps you adjust your game strategy.
Now when you don't have you're best stuff, these are going to be the tough days, but your mental approach can still keep your team in the game.
You should realize that you still have the ability the get hitters out, so you've got to keep your confidence up and pitch a smart game.
You've still got to throw strike one and get ahead in the count. If your fastball is a little short that day, keep it down in the zone and try and get some ground ball outs. This is where your mental toughness is going to come in. You're still going to have a game strategy but it will have to be adjusted to you not having your best stuff. If you realize that only you knows what pitch is coming, you'll stay more focused and confident that you can get the hitter out.
Remember it really doesn't matter how good your stuff is on any given day. With a flexible and well thought out game strategy, you can still be successful.