How to achieve the top score playing drums (solo) in Rock Band 2

One more thing is required to maximize your score (in addition to 100% accuracy and optimal overdrive pathing that the other posters mentioned) -- you need to squeeze. Squeezing has essentially two methods that allow you to get credit for an additional drum hit.

• Play the 'hidden note' underneath the overdrive activation (green pad). Well, it's not actually hidden ... it's the note that would normally be charted in that spot if there was no overdrive activation sequence. So one plays the green overdrive activation note slightly early (but still within the timing window) and then immediately play the note (or notes/bass) sequence that would normally be charted (quickly! it needs to be within that same timing window) ... and you get the usual credit for that hit (25 * #pads * multiplier). This works for all overdrive sequences.
• Exploit the timing window around overdrive. Essentially you start the overdrive as late as possible in the overdrive activation window - and then if there's a note would usually be just past the overdrive window, you now have a brief opportunity to play the note slightly early (but still within the activation zone). Voila ... credit for one additional note. Think of it as exploiting a rounding error. Note that this is dependent on the charting of the song .. there needs to a note in the correct place for this to work, so not all overdrives can squezee a note here.

These techniques don't earn many points (essentially one extra note or notes each squeeze) and can be quite intricate to time ... but are essentialy to earn the very maximum score (greater than merely 100% FC and optimum overdrive usage). Note that a similar tactic is usable on guitar/bass, and is also useful in Guitar Hero as well as Rock Band.