It sometimes strikes me as amusing that the amount of anguish students experience in their pursuit of tremolo technique is so out of proportion with the actual volume of tremolo repertoire we have. And yet, it is clear that the technique applies broadly to the playing of arpeggios, and has salutary effects on right hand control in general, so I always encourage careful attention to this technique.
Tremolo is a big subject, so I will write several posts addressing different aspects of practicing and performing it. This first one describes the basic, entry-level workout. The basic practice routine I recommend follows.
Place the left hand on a simple diminished triad on strings 4-3-2 in the 1st position (from 5th string to 2nd, the pitches are a, d#, a, c). The tremolo is played on the 2nd string while the thumb plays the three lower pitches in alternation. My experience is that, if you can control tremolo there, it'll be easy on the 1st string.
Treat this chord as a moveable construction and repeat the exercise with it, ascending, in every position through the 11th. Play four tremolo patterns on each fret, corresponding to four alternating bass notes, in 4/4. Play with metronome on, rigorously in time.
Of critical importance:
1) play with p extremely softly and with the fingers both extremely loudly and as evenly as possible. Maintain that exaggerated disparity in volume throughout.
---In nearly every piece in which tremolo is featured, the tremolo is melodic and the bass notes accompanimental. Conversely, in inexperienced hands, the opposite it what we usually hear. Strict and exaggerated dynamic contrast overcomes this flaw.
2) prepare each note the instant the previous one is played. This will cut the first two tremolo notes very short but let the third ring as the thumb prepares.
---The most terrifying thing about tremolo to most students is it's speed. The fast preparation helps facilitate a fast goal tempo.
Once the left hand reaches the 12th fret in each iteration of the exercise, head back down the neck, back to the first position, legato and double-time. So, play for half the time with a sharply staccato-preparation, and half the time as legato as possible; and, for half the time slowly and half the time twice as fast.
3) repeat the excercise with each of the following seventeen patterns:
pami (use in pieces)
piami (5-note tremolo)
pimami (6-note tremolo)
--It is the natural movement of the hand to close the fingers to the palm, weak-side-first. Therefore there is a comforting naturalness to the pattern we use on stage, p-a-m-i. Unfortunately, it is that very comfort which often leads to a gallop in the tremolo's rhythm: it is easy to close the hand (play a-m-i) fast but doing so in a rhythmically even manner takes great control. The best way to gain that control is to master all the finger combinations, thus gaining real finger independence.
Of course this list of patterns is too much for a daily routine, so I recommend doing three patterns/day each week: always practice a-m-i, and also practice two additional patterns from the list. The 16 remaining, at two/week, adds up to an eight-week routine to master them all, while in a daily workout on a-m-i all the while.
Practicing like this is foundational. It is the type of work we do to become classical guitarists. It takes time for new routines to become internalized, but it happens gently and naturally with daily work. The routine as described takes about 10 minutes. I've found it's time well-spent!
(Note: thanks to David Russell for some elements of this routine, especially the notion of practicing the same finger in repetition (p-i-i-i, etc.). I'll never forget, as a student, watching him demonstrate a highly convincing Recuerdos using only the i finger).