In many cases, pieces that are recolors of the same design can usually go well together.
For example, the Veteran Armor, Veteran Leggings, and Banded Pauldrons make for a nice combo with the Soldier's Gauntlets and Soldier's Boots, and the Nexus-Strider Belt, because they are all recolors of the same set of mail.
As a good rule of thumb though, so you don't get too elaborate by picking bits from a half-dozen recolor sets, remember 3-2-1. Three items from one set, two from another, and one from a third. That usually offers a fairly good range without overcomplicating things.
Like in the example above: the chest, legs, and shoulders (3 items) are from one set, the gloves and boots (2 items) are from a second set, and the belt (1 item) is from a third. This spread of pieces (chest/legs/shoulders, gloves/boots, and belt, are also typically the best way to arrange the pieces, though sometimes arrangements like shoulders/boots/belt, gloves/legs, and chest, can work too - feel free to experiment, since not all recolors work the same way.
This doesn't take into account things like helms and cloaks, mind you, since those can be (and often are) turned off if/when they don't quite fit a look, but this is still some fairly useful advice anyway, I think.
