Ever come to a point where you feel you've learned enough to share your experiences in the hopes of helping others traveling the same road? That's what Martin Kleppmann has done in an lovingly written Six things I wish we had known about scaling, an article well worth your time.
It's not advice about scaling a Twitter, but of building a million user system, which is the sweet spot for a lot of projects. His conclusion rings true:
Building scalable systems is not all sexy roflscale fun. It’s a lot of plumbing and yak shaving. A lot of hacking together tools that really ought to exist already, but all the open source solutions out there are too bad (and yours ends up bad too, but at least it solves your particular problem).
Here's a gloss on the six lessons (plus a bonus lesson):