How do I 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.

It varies widely by the song, but it's all about using (or not using) overdrive at the right time.

The thing that makes it tricky on drums is that whenever you have the ability to use overdrive you get chunks of the song where you can score no points at all (the fills).

In solo mode most of the time you just want to use it every time you have the chance, to minimize the amount of notes that are removed from the game by the fill sections. The only time that it might be worth it to save it for the next fill is when you know the following section will either be fewer (or worse: no) gems than after the next one, or if there's a tricky fill in the song that you're likely to miss a note and break your streak, the overdrive deployment fills usually cover these up for you, so it's worth it to miss out on a few gems if you get to keep your multiplier.

If you're playing with the rest of the band, that's when it gets quite a bit more complicated as you'll have to consider all the instruments' parts as well as the fact that being able to keep the overdrive going for a longer period of time (stringing them together as long as possible) could be worth a lot more than the gems you don't get to hit.

Zemm + Didi's advice is exactly it. To get you started, here is a site of optimal overdrive activation paths and the notes you will need to squeeze for extra points. This has all the DLC and different games, too, in separate categories.