Enchanting two normal’s requires 1 stone, enchanting two +1's will require 2 stones, and enchanting two +3's will require 4 stones, and so on and so forth. Enchanting never fails from what I gather eve online isk and I've never see it fail personally, so I doubt it can. This is the most common method in the game to obtain better items.
When you 'enchant' something, it melds some of the items stats together to produce a higher quality item. Also remember, use corresponding stones, weapons for weapons, armor for armor, accessory for rings.
Once you get past the Beginner Enchant stones (and boxes), the cost for the stones becomes important. Currently, a (Beg) box seems to be running around 800, while an (Int) box seems to be running around 8,000; this is a pretty harsh factor of 10 increase. Consider that a +10 item not only requires 1,024 base +0 items, but 2,036 stones.
I expect the cost of the higher boxes / stones may go down some as the game matures, but how much is an open question. In combination with the above, this leads to it generally being more effective to keep enchanting the items just before the break.
A +4 Ogre Bow is more effective in every way than a +2 Assassin's Bow, but usually cheaper to buy or make, and you can start using it ten levels sooner.