Well, my own understanding is that "rationality" is the art of being rational (of using reason and its related tools to improve your map of reality, and therefore be able to steer the future in a direction you desire) while "rationalism" is the philosophy that says using "rationality" is a good thing.
For example, a post that explains what Bayes' theorem is and how to use it, what is the "fundamental attribution error" and how to avoid it, or explaining the basis of game theory are "rationality". A post that explains the importance of "wanting to be stronger", how you shouldn't accept blissful ignorance (aka "doublethink") or why we need to "raise the sanity waterline" are "rationalism".
That's the difference between saying "how you do it" and "why you should do it".
Of course in many cases, the limit isn't clear. Words are fuzzy boundaries, after all.
I feel that the term "rationalism", as opposed to "rationality", or "study of rationality", has undesirable connotations. My concerns are presented well by Eric Drexler in the article For Darwin’s sake, reject "Darwin-ism" (and other pernicious terms):
So, my suggestion is to use "rationality" consistently and to avoid using "rationalism". Via similarity to "scientist" and "physicist", "rationalist" doesn't seem to have the same problem. Discuss.
(Typical usage on Less Wrong is this way already, 3720 Google results for "rationality" and 1210 for "rationalist", against 251 for "rationalism". I've made this post as a reference for when someone uses "rationalism".)