I'm not a real Guitar Hero guru, but I already completed the career mode of World Tour with 100% at difficulty hard (well, I have to admit that I nealry did not get through the hardest songs). So I want to tell you some techniques which I believe to be very helpful. But of course you have to see which techniques are the best for you.
Positioning of Your Fingers
There are two possible base positions of the fingers: Either you have the index finger at the green button or at the red button. The other fingers are in their order at the lower buttons.
At the difficulties below hard the position with the index finger at the green button is probalby the easier one, because at theses difficulites only the first four colors occur. But from hard onwards you should have the index finger at the red button. The reason is that the index finger is much more agile than the pinky. You can move your index finger faster between the red and green button than the pinky between the blue and orange button.
Who wants to learn it right from the beginning should also use this finger position at the lower difficulties. Moreover, it is often not very easy to relearn such sings. Anyway, it can also be very useful to be able to change between both positions fluently. Especially the last songs in Guitar Hero III: Legends of Rock this would be very useful, because there are sequences with fast changes between green and other colors, but not orange.
It's a pity that I am not able to do this change. The only exception are the chords including green, because this can be much easier played if you move the whole hand position one button up. But this is much more easier for chords, because you play these always with the same hold. For the single notes you would have two holds for each and you had to chose more or less automatically the right one.
I am using three different techniques for triggering and I change between them also in one song as needed.
With the Thumb
Probably the most easy triggering technique, which I used always at the beginning, is the triggering with the thumb from the top. Even now this is my standard technique. It is most useful if the notes or chords to play are always changig or the notes are not all in the same distance/beat.
You have a disadvantage with this technique if very long and fast note series appear. Firstly, the thumb tires out relativly fast if you have to trigger very often. Secondly, the thumb is not the fastest finger.
With the Index Finger
Occurs the same note in a long row with a short distance, the best method to trigger is to use the index finger from below. This finger tires not as fast as the thumb out by a long note series, which occur often in Guitar Hero World Tour for the bass. If needed you can also change to the middle finger. Moreover, the index finger is a little bit faster than the thumb and you can trigger all the notes without problems even if they are in a short distance. In my opinion it is easier to hold the beat with the index finger than the thumb as long as the beat is regularly.
With the Thumb and the Index Finger
The third triggering method is to hold the trigger between your thumb and index finger and to move it alternating up and down. With this technique you can trigger really fast! Some note series have so many notes that you can hit them only with this triggering method. But also with slower series this method is comfortable. However, if the distance between the notes gets too long, it gets very difficult to hold the beat, in my opinion. If this is the case I am far better off using only the index finger.
This technique is most useful, I think, if some notes occur in a block consisting out of two, three or four notes. If a block consisting of two notes occurs you move the trigger down and on time up – finished. For a block consisting of three notes: down, up, down. In other words you are finished when you are back down. Accordingly you are finished with a block consisting of four notes when you move the trigger the secon time up. With this method you can easily make sure not to trigger too many or too less notes.
Often you have a series of notes in the same distance (which can be triggered with the thumb) with blocks consisting of two, three or four notes in a shorter distance every few notes. If you combine the thumb technique with the thumb-and-index-finger technique these passages can be played very well. You trigger the notes with the thumb in one single beat and continue this triggering movement moving the trigger up and down to hit the notes of such a block. The thumb is than practically always triggering in the same beat. You just use another movement to trigger which triggers than the whole bloc.