Take a look around the threads here. One important one is the Data Sheet Administration. There are threads dedicated to leveling armors. I'll try to give a brief run-down:
Every armor is assigned a certain number of "enhancement points". It has one value for non-matching elements and a slight bonus if the elements match. Generally, the higher the level of the armor, the higher the enhancement points it will give. The required number of total EP required to level an armor depends on it's max level, e.g. 1/30 or 1/70.
Basics provide very little EP per armor. Uncommons (Stonescale, Dragonflame, etc) provide a little more, etc.
Every level of the armor above level 1 provides another point of EP. For instance, Stonescale armor provides 10 EP. A level 11 Stonescale provides 20 EP (level 11 - 1 = 10 extra EP). You can get uncommons to level 11 by fusing (enhance) 8 basics into it before using it as the fuel for an ehancement.
Snakeskin is popular because it is relatively easy to farm, matches two elements, and has a lower cost than most 1 star armors to craft. Snakeskin provides 20 EP for non-matching armors and 24 for matching armors.
Basics and uncommons are not very good fuel for enhancements. Raising uncommons to level 11 before using them makes them as effective as snakeskins if the uncommon is matching and the snakeskin is not. However, those uncommons will cost more gold to use because you do have to raise them to level 11 first.
For level 70 and 99 armors, there comes a point when you will probably want to fuse lower level armors to make 50 EP armors to use as enhancement materials. There is a thread here that discusses what you can fuse to get guaranteed 50 EP armors. These are obviously costly to craft for leveling the armors. However, the cost of enhancement goes up with the level of the armor. For instance, the enhancement cost when the armor is 1/70 will be 150 gold per piece added. At 50/70, that cost will be MUCH higher. Therefore, using 50 EP armors becomes effective because you will be paying the enhancement cost fewer times.