Even if you don't want to play 'Recuerdos de la Alhambra', this is a great exercise for the RH: it improves flexibility and strengthens muscles.
Once you have developed a good tremolo, it is a good idea to keep it up as part of your regular routine as the evenness and strength will disappear over time. However, it does come back quicker than had you never practised it.
More than any other technique, the slightest change in hand angle or sitting position will effect the balance and evenness of your tremolo, so try to keep it consistent.
Also, try it staccato too, which will help with speeding up.