I figured out that if you press all buttons at the same time in rythm of the music on medium difficulty it won't affect your grade.(this doesn't work on the hard difficulty)
This method will allow you only to concentrate on your timing.
I also recommend like said above to remove your system cache so you can get an A* with a 188% combination instead of a 192% combination.(So you need a 100% hit ratio and at least 88% accuracy)
If you want the time stamp on your chievo just save one easy song so you can do this one online.I recommend saving Sting of the Bumble Bee or Hall of the Moutain Dude since this songs are the easiest ones.
So just try to get as many perfects as you can.Try only to get perfects or Awesome's.If you get too many Ok's retry the song cause at the end you will only get a B rating.
Score doesn't affect the final rating so don't press RT and LT to get the bonus points.It will only make it harder.
It's all about Training and Timing so don't give up and you will achieve this !
[ Songs on wich i got an A* : Smooth Operetta, Rave New World, Hall of the Mountain Dude, 1812 Overdrive, Valkyries Rising, Tail Light Sonata, Carmen Electric, Toccata and Funk, Sting of the Bumble Bee (DLC) and Explode to Joy (DLC) ]
Hope this helps and good luck !