For me it depends on the style of the game.
I'm with Maddman on this one.
I don't think there's any possible way to objectively define "skillful play". Even a merit system will include the elements that the DM subjectively considers valuable as indicators of skillful play.
I think of skillful play, like "skillful" anything, in this way - Few people may be able to define it linguistically in-book by a rule-set but almost everyone recognizes it, and its opposite, in action.
Crazy idea for mature bunch of players:
At the end of the adventure, give each player a fixed amount of xp which then can award to other PCs (not their own) as a sort of MVP/most entertaining player moment.
or to bring up to par a PC who is languishing with lower hp
Nice Idea
Of course, I'm under the impression, and have used this technique for a long time, that rewards should be far more diverse than treasure taken, and/or experience points gained for action.
In real life there are a multitude of rewards, for instance, which characters could also easily achieve. Reputations, marques and honors, estates, titles, gains in abilities, capabilities, and skills (through exercise and practice - few things are better exercise and practice than actual field-work), gifts, legacies, rewards such as political power, information, and so forth and so on. I think that the game, any game, should have a wide range of "rewards" and methods of gaining value and advancement, rather than just a small number of limited means of "reward." Even intangible rewards, it seems to me, should have some effect as to "character advancement." And I'm using "advancement" in the widest sense possible.
Of course most of those things are setting and milieu rewards, and most wouldn't work well or have any real value in hack and slash games, or one shots.