A very useful column from Yahoo’s Christopher Null. The key graph:
The trick is to use a “root” that is not in that list that I linked above, and to put your “appendage” (or two of them) in an unusual place: Either in the middle of the root or at both the beginning and the end.
Schneier’s example is to use a word that you can pronounce but which is spelled “wrong”: armwar or pitchsure or baysball are all examples. Then attach your appendage(s): arm9!9war or 1066pitchsure6601 or bay1776sball. It shouldn’t take much effort to commit any of these to memory.