I find sometimes changing a gun skin makes me play better with that gun. For example, the Openarena shotgun is long, thin and precise, and pokes out of side of my screen. The Quake 3 shotgun is wide, short and placed closer under my nose. I more confident with the Quake 3 shotgun, as it just feels as if the gun model suits the nature of the gun better, while the Openarena shotgun looks too precise and fiddly, and I have difficulty aiming with it.
Also, I literally could not use the Deagle from CS:S for shit, but I changed the gun model to a Boberg pistol, and suddenly I became a Deagle demigod. The Boberg looks chunkier and shorter, and I prefer it.
I can't really explain it, but it works.
Most times I just like to dick around though.
http://www.fpsbanana.com/skins/62795