Ok let me try ...
The key to consistency is your heart & your fundamentals. To the topic starter, u mentioned that 1 day you are good & on another day you're playing like hell. Keyword here is 'day'. Now we dont want to be inconsistent the whole 'day'. What you should do is to shorten that whole 'day' into just a few shots. We want only 'short term inconsistency' & not a whole day of inconsistency. If your inconsistency is not because of fatigue or mind problems, then I can help you:
Putting Fundamentals Into your shotsWhenever u made a bad shot, u need to look back & see what u did wrong in that shot. U cannot see & know what u did wrong in your shots if u dont have stroke fundamentals. What is stroke fundamentals? Stroke fundamentals are the blueprint & step by step basis to a stroke process. For example, we take a normal forehand groundstroke & we break the fundamentals down to a step by step process, it would become something like below:
1. Tracking of the ball from your opponent's side of the court
2. Watching & interpreting the flight path & trajectory of the ball
3. Process that in your brain & make a judgement on where it would land & how high the ball would roughly bounce
4. Once your brain has calculated on where you should move to take that shot, your physical body does the moving
5. You step in with the correct foot & taken your racquet back. Your eyes tracking the ball continuously.
6. You contact the ball in front of you at the right striking height & distance, your racquet is not closed or opened face, it's parallel to the net & your eyes is locked on the contact point
7. You pull your racquet across your body, accelerating from the point of impact, bringing your racquet along a crescent like path across your shoulder
8. You finished your stroke with your elbow pointing to the opponent's court & your non hitting hand catching your racquet's neck on the other side of your body.
Depending on where/how/who u learnt tennis from, your fundamentals might not look like the above or you could even have totally wrong fundamentals if you learned it from a bad source. So before you even go into consistency talks, u need to know whether you have the proper fundamentals or not. To do this, you need to have a very reliable & skillful coach that can put fundamentals into your strokes by not only teaching you to hit physically but he must also be able to put all his teaching onto paper. A good coach is a coach that goes the distance in letting the student understand fully & implanting written fundamentals into the student's brain.
Now why I tie fundamentals with inconsistency? Answer: U are inconsistent because u hit bad shots. Why u hit bad shots? Answer: Because u messed up part of your stroke fundamental's process. Believe me, every ball that u hit wrong, it's because u messed up some part of the steps in your stroke process... every single time. If u look at the pros, sometimes when they hit a bad shot, they would retake that shot by doing a shadow swing after that mistake. What they're doing is to repeat that stroke process again in a correct way. They're looking back at their mistake & they're understanding what they did wrong. They have fundamentals, they know what they did wrong, they're psyching themself's up so that they wont make that mistake again. U need to have proper fundamentals so that u can know what u did wrong & stop doing it the next time.
Putting your heart into the game with positive emotion/temperamentThe second key to consistency is your heart, which is pretty straight forward. You dont play tennis if you're bothered with other things that day. What are 'other' things? Well... things like:
1. I haven't eaten today
2. I dont have enough sleep last night
3. My opponent is a state champ
4. I'm injured
5. My racquet is not good
6. My mom is sick at home
7. My head is thinking of a Xmas turkey, Dom Perignon & Megan Fox
Playing good tennis requires a lot of heart, focus & concentration. If you're so bothered by other things, you better not play that day & use them as reasons. Dont play with bad emotion ie: anger, frustration, sadness, etc. Emotion & temperament are big subjects, you should read up some mind control advice for this. Anyway, we play tennis because we love this sport. Nobody is forcing us to play, we play this game willingly. There is no reason why we cant put our heart into it. I dont need to elaborate further on this.
SummaryTo sum this up, here is what you should do:
1. Play the game stroke by stroke, not game by game or set by set. Dont think on what you want to do with this set, think of what u need to do on this stroke so that u can break down your fundamentals & go step by step in executing that stroke. Remember, a stroke must be executed with a set of process, messed up part of the process & it will turn into a bad shot. Bad Shot = Inconsistency.
2. Once u made the first bad shot, recognize the mistake & dont repeat it again. You want the inconsistency period to be short. We dont want long term inconsistency. Slowing down your game after u've made a bad shot would help.... when I say slow down, that means u should execute your upcoming strokes with more care & make sure u execute the stroke step by step with full fundamentals. Get back that stroke rhythm slowly & build back pace.
3. Put your heart, focus & concentration to your execution. Dont let anything else bother u.
4. If you're fatigue & got mind problems bothering u, dont play. Go back & rest.