One of my best students recently asked how he could further improve his philosophical work. It's something I occasionally wonder for myself, too. So I thought I'd throw the question out there for the collective wisdom of the Internets to answer: how should one "train" in philosophy?
Most obviously, one can learn from expert feedback on one's written papers. One's instructor may pick up on the main flaws or possible objections that should be addressed, and suggest potential avenues for further developing and extending the core ideas of the paper, which should eventually help one to develop a "sense" of what makes for good philosophy. However, there may be only limited opportunities for obtaining such formal written feedback (since professors are likely busy with their own work, and perhaps reluctant to do anything that feels like additional "grading"), in which case more general, informal discussions -- e.g. in office hours, or by email -- may be more welcome.
More generally, simply doing philosophy -- writing, and arguing with others -- is presumably always good practice. My advice: start a philosophy blog, and use it to (i) practice clearly summarizing any interesting ideas you come across in readings and lectures; and (ii) test out some of your own arguments and ideas. (At least, I find this invaluable. YMMV.)
Then there's the method of simply exposing oneself to a lot of professional-level philosophy: read - or skim - the top journals, attend departmental talks or 'colloquia' by visiting professors, etc. See if you absorb their philosophical skills by osmosis; or, better yet, combine with the previous suggestion to actively engage with the new ideas and methodologies you encounter.
(These procedural questions aside, there's also the question of what philosophical content to focus on. I would strongly advise undergrads, especially, to obtain a broad familiarity with the various sub-fields and methodologies: from ethics to formal logic, and from armchair analyses to more empirically-minded approaches. Further, I think almost any philosopher can benefit from thinking about the connections between their field and others -- e.g. between ethical and epistemological normativity, or between meta-ethics and meta-metaphysics.)
Have I missed anything important? And how would you balance the various recommendations -- is a marginal hour better spent reading more journal articles or developing more of one's own ideas, say?
[Cf. Getting the Most Out of Grad School]