Not sure if you have online access, but make sure you have the latest tuner update downloaded, it should download automatically when you start the game while connected to Live or PSN. I see a decent amount of unforced errors from the CPU on Hard and Very Hard after the most recent update. If you are playing it out of the box with no tuner updates there are very few unforced errors.
What it comes down to more than not is getting the hang of timing your swings to get "Good" or "Perfect" timing on most of them. This will allow you to place your shots where you want and get the maximum power on charged swings which is one of the keys you will need to win.
Some other tips
-Learn to read what kind of shot is coming back at you and how to deal with each type.
- If it is an easily returnable ball, most times you want to hold down the normal button and power shot it back.
- If it is a ball you have trouble getting to, tap any button rather than charging it to help ensure you dont return it out of play.
- Also, pay attention for the slice shot, as you want to avoid trying to return these with power shots as that often causes you to hit it out.
-I always try to return serves to the opponents backhand side so they cant follow up with a power accurate forehand winner. The slice shot works pretty good as a return for tough to get serves. Otherwise try charging the normal button and releasing it earlier than usual to get good timing on returning serves.
-It helps when trying to place shots to either side to also hold down to create a tougher angle for the CPU to deal with.
-Play to your players strengths and exploit your opponents weaknesses. Check the stats for your player and the CPU before the game and try to tailor your play style a little towards what they are good at.