Practice. As well as trying to notice when things work or don't (I have colleagues that see "firefighting" as a normal part of a running system. If something keeps needing attention, it is probably badly written or architected).

Linus said something along the lines of bad programmers worrying about the code, and good ones worrying about the data structures. I think there is truth in this, even though I develop Django database applications most of the time. If I find myself doing too much work at the application level, taking a step back I can usually see a better approach when I rearrange the database.