Honestly, one thing I've noticed on PMC is the lack of self-esteem when skinmaking.(feeling a cliche, anyone?)
I, savior of cats, have come to (slightly) remedy the poor situation. The thing is, is that people just make a skin with three colors, and then upload it without a second thought. Lather, rinse, and repeat. Sure, these people are pumping out skins like nobodies' business(okay, only a few are), but they look like crap! People just neglect shading and put up their skin and want downloads, and so, they get a few.

On to the point. Shading! I may not be the best at it, I may seem a bit like a hypocrite, but it's a little touch that can mean the difference between Blegh and OMG!That's Awesome! All you need to do is add a few contrasts on edges, maybe give a few scuffs or tear the jeans on that new teenage boy skin(rant for another time) and it'll be unique and, most of all, good! I like making the elbows on a person's long-sleeve shirt a bit more worn, as it makes it a bit more realistic. So, people, just take my(and half of PMC's) advice and put more effort into your skins!

Skinmaking is one of the things where quality beats quantity!