I. For those just starting, the best teachers of philosophical writing are the Brothers Feldman.
For explicit instructions, useful exercises and two extended examples of how to write a philosophy paper, see
Richard Feldman, Reason and Argument. (1st or 2nd. eds.)
Then, to see this advice in application, study the pellucid and rigorous
Fred Feldman, Introductory Ethics. BJ71 .F44 (2nd ed. soon to be forthcoming?) In Reserve Room - D.H. Hill Library
Tom Regan also recommends:
Bertrand Russell, The Problems of Philosophy BD21 .R8
James Rachels, The Elements of Moral Philosophy BJ1012 .R29 1993
II. Now you're ready for somewhat more advanced material:
Fred Feldman, Confrontations with the Reaper (on the natures of life and death; includes discussion of abortion and euthanasia) BD444 .F44 1992
John Perry, "The Problem of Personal Identity," in John Perry, ed., Personal Identity, 3-30 BD331 .P46
Alvin Plantinga, God, Freedom and Evil UNC/Davis BT102 .P56 1974
Brian Skyrms, Choice and Chance 3rd ed. (the undergraduate text on inductive reasoning)
III. You might want to try the more difficult
Ned Block, "Psychologism and Behaviorism," Philosophical Review (1981) B1 .P4
[at least one by Chisholm:]
Roderick M. Chisholm, "Editor's Introduction," Realism and the Background of Phenomenology B835 .C5
Roderick M. Chisholm, "Intentionality and the Theory of Signs," in H Feigl, W Sellars and K Lehrer, eds., New Readings in Philosophical Analysis B808.5 .F45
Roderick M. Chisholm, "Law Statement and Counterfactual Inference," in H Feigl, W Sellars and K Lehrer, eds., New Readings in Philosophical Analysis B808.5 .F45
Carl Hempel, Philosophy of natural science Q175 .H423 (older, but elegant and good)
Alvin Plantinga, God and Other Minds BT102 .P55
Israel Scheffler, The Anatomy of Inquiry (an older text in philosophy of science, with lots of still-important arguments) Q175 .S33
IV. For the most advanced and determined undergraduate, these are worth a try:
Richard Cartwright, "A Neglected Theory of Truth," in his Philosophical Essays, 71-93. (a fine introduction the problem of false belief - but the last section is pretty technical) B29 .C2785 1987
Carl Hempel, Aspects of scientific explanation, and other essays in the philosophy of science Q175 .H42
Saul Kripke, Naming and Necessity (the most important work of contemporary metaphysics and philosophy of language, by the world's smartest philosopher) BD417 .K74
Saul Kripke, Wittgenstein on rules and private language: an elementary exposition (-a brilliant exposition of Wittgenstein's 'Private Language Argument'; presents a very deep puzzle about the nature of thought and meaning) B3376 .W564 K74 1982
Harold Levin, "Justifying Symbolizations," in Judith Jarvis Thomson, ed., On being and saying: essays for Richard Cartwright B29 .O67 1987
David Lewis, "Causal Explanation," in his Philosophical Papers Vol II 214-240 B29 .L49 1986
David Lewis, "Paradoxes of Time Travel," in his Philosophical Papers Vol II 67-80 B29 .L49 1986
David Lewis, "Veridical Hallucination and Prosthetic Vision," in his Philosophical Papers Vol II 273-290. (a series of clever and amusing examples that illuminate questions about skepticism) (see also Bruce LeCatt, "Censored Vision," Australasian Journal of Philosophy 60 (1982) 158-62.)
Israel Scheffler, Beyond the letter: a philosophical inquiry into ambiguity, vagueness, and metaphor in language P106 .S273
Peter van Inwagen, An Essay on Free Will BJ1461 .V36 1983
V. How to Do History of Philosophy
Jonathan Barnes, The Presocratic philosophers B188 .B34 1982 (Barnes sometimes gets a bit carried away by his enthusiasm for formalizing arguments.)
Terence Irwin, Plato's moral theory: the early and middle dialogues B398 .E8 I78
Daniel Garber, "Motion and Metaphysics in the Young Leibniz," in Michael Hooker, ed., Leibniz: critical and interpretive essays B2598 .L435 1982
Michael Hooker, "Descartes's Denial of Mind-Body Identity," in Michael Hooker, ed., Descartes: Critical and Interpretive Essays B1875 .D38
George Pappas, "Berkeley, Perception and Common Sense," in C. M. Turbayne, ed., Berkeley: Critical and Interpretive Essays B1348 .B44 1982
Robert C. Sleigh, Jr., Leibniz and Arnauld: a commentary on their correspondence B2599 .M7 S53 1990
Robert C. Sleigh, Jr., "Reid and the Ideal Theory on Conception and Perception," in Stephen F. Barker and Tom L. Beauchamp, eds. Thomas Reid: critical interpretations, B1537 .T48
Margaret D. Wilson, Descartes B1875 .W58
Margaret D. Wilson, "Did Berkeley Completely Misunderstand the Basis of the Primary-Secondary Quality Distinction in Locke?," in C. M. Turbayne, ed., Berkeley: Critical and Interpretive Essays B1348 .B44 1982
R. S. Woolhouse, "The Nature of an Individual Substance," in Michael Hooker, ed., Leibniz: critical and interpretive essays B2598 .L435 1982
VI. Readings that concentrate on ethics
Joel Feinberg, "The Nature and Value of Rights," Journal of Value Inquiry 4 (1970) 243-257 BD232 .J65
Thomas E. Hill, Jr., Autonomy and Self-Respect BJ1533.S3 H55 1991 (UNC/Davis)
Thomas E. Hill, Jr., Dignity and Practical Reason B2799.E8 H55 1992
Judith Jarvis Thomson, "In Defense of Abortion," Philosophy and Public Affairs 1 (1971) 47-66 H1 .P54
A. Donald VanDeVeer and Tom Regan, eds., Health Care Ethics R724 .H34 1987 (see essays by Brock, Buchanan, Sumner)
VII. Some of contemporary philosophy relies on material from mathematical logic. If you have the necessary technical background, then these writers and works provide excellent examples of how to write about philosophical logic and philosophy of mathematics:
Charles Chihara, Constructibility and mathematical existence QA8.4 .C45 1990 (Part II is an especially clear discussion of the contemporary debate about the nature of mathematical objects.)
John Etchemendy, The Concept of Logical Consequence BC135 .E83 1990
Paul Horwich, Truth BD171 .H67 1990
All of the philosophers mentioned here are exceptionally clear writers with styles worth emulating.
Beset by self-doubt? Not sure if philosophy is for you? See the Special Little Philosophy Talent Test!