(1) Reassure them that nothing 'spooky' is required. (I assume such fears are what motivated them to banish the concept in the first place.)
(2) Offer cognate terms. Scrap 'morality'. Try talking about what you really ought, all things considered, to do. What matters. Or consider what would be best, or what you have most reason to do; what's reasonable, or rational.
(3) Quote Sidgwick (1890: 39)
Even, finally, if we discard the belief, that any end of action is unconditionally or "categorically" prescribed by reason, the notion 'ought' as above explained is not thereby eliminated from our practical reasonings: it still remains in the "hypothetical imperative" which prescribes the fittest means to any end that we may have determined to aim at. When (e.g.) a physician says, "If you wish to be healthy you ought to rise early," this is not the same thing as saying "early rising is an indispensable condition of the attainment of health." This latter proposition expresses the relation of physiological facts on which the former is founded; but it is not merely this relation of facts that the word 'ought' imports: it also implies the unreasonableness of adopting an end and refusing to adopt the means indispensable to its attainment.
(4) Go procedural. Consider the possibility that a reflective equilibrium process might lead one to change their values/preferences in ways that could only be described as an improvement. For example, one might iron out any inconsistencies, reduce the number of ad hoc/arbitrary distinctions, add more general principles that enhance the overall coherence and unity of one's desire set, etc.
(5) Offer examples of irrational preferences, e.g. future-Tuesday indifference, only caring about your future self's interests up until 1/1/09, altruistically caring about all and only persons whose names begin with the letter 'A', etc. Intransitive preferences. Failure to pursue the necessary means (ceteris paribus) to some endorsed end. Preferring the acknowledged lesser good to the greater. And so on.
(6) Draw attention to their agency. These skeptics usually presuppose a kind of naive Humeanism, according to which preferences are 'given' and automatically combine with beliefs to yield action. But that can't possibly be right, because it leaves no room for the familiar phenomenon of deliberation. We are agents with the capacity for practical reasoning, i.e. the assessment of reasons that count for or against various courses of action. This is a self-consciously normative process of decision: just as theoretical reasoning addresses the question what should I believe?, so practical reasoning addresses the question what should I do? Insofar as you think of yourself as a rational agent at all, you must be engaging with these normative questions; the alternative is to be a mere automaton, a reflexive stimulus-response machine. Most of us are more deliberative; but deliberation is inherently normative: it addresses a question for which there may be better or worse answers.
(7) Compare epistemic normativity. For some reason, people seem to be more skeptical of practical reason than theoretical (epistemic) reason. Even the most hard-nosed science-cheering skeptic usually thinks that Creationists, say, are going wrong in their beliefs. This is not just to say that their beliefs are likely false, or that they are unsupported by evidence (though this is part of it); in response to someone who invokes practical reasons for belief (say religion makes them happy), the skeptic may make the further claim that they are being unreasonable. (Cf. Sidgwick in #3 above.) Practical normativity is like that, only applied to actions rather than beliefs. Performing bad actions is kind of like believing contradictory things. People manage it all the time, but they're cognitively malfunctioning in doing so.
An additional point of analogy: skeptics may initially be inclined to an inadequately narrow conception of rationality. Deductive logic gives us no reason to believe that the Sun will rise tomorrow, as per the famous problem of induction. But we clearly do have good reason to believe this, and indeed it would be unreasonable not to. This points the way to a broader conception of rationality which invokes considerations of coherence, etc., much as Future Tuesday Indifference and similar examples (#5 above) show the need to go beyond mere instrumental rationality in the practical sphere.
Any other suggestions?