What is the main factor for levelling each of the 18 skills in Skyrim?

PICKPOCKET: it's value that counts. for levelling. Weight only affects how likely you are to get caught. Value ALSO affect getting caught - which is why it's virtually impossible to steal large amounts of gold or very expensive jewellery at lower levels.

to start off with pickpocket, steal gold, and arrows (0 wieght, so almost no chance of getting caught). when u get a few levels, start stealing jewellery a ring or necklace wirth 50-100 will easily take u up a while level in the teens-20s. once u get to level 30+, you'll be looking at around 200 value for a full level increase

SPEECH: value of item sold. NOTE if you have for instance, 10 swords, selling them all at once will only count for the value of 1 sword - so sell them seperately. successful persuasion, bribery and threaten attempts will also increase speech

LOCKPICK: failing to pick a lock (broken pick) increases skill slightly, successfully picking increases much more, and obviously the harder the lock, the bigger the increase.

SNEAK increases whenever you are sneaking within range of someone who would otherwise have noticed you (sneaking when there's no-one around doesnt do anything)You will increase MUCH quicker if you do something while sneaking - for instance, sneak attacks. Sneak attacking wildlife with your bow levels you really quickly

ARCHERY successful hits with your bow level you, so again, attack wildlife everywhere for quick levelling

SPELL schools are the same for all 5: a successfull spell increase your skill level. Note, the spell must have an effect - casting a desruction spell on a rock does nothing, casting it on an enemy does.

ENCHANTING: i think the best way is to enchant/disenchant weapons at the arcane enchanter, but a much cheaper and still very quick method is just to simply recharge magic weapons with soul gems. i have never yet enchanted a weapon, yet i have level 35 enchanting just from charging magic weapons

SMITHING forget daggers, gold and silver rings give you a slightly higher increase, are worth much more, and gold and silver ore are as easy to mine as iron ore. also, if u have visited a dwemer ruin, 2 dwemer ingots plus one iron ingot makes a dwarven bow worth 270 gold (which is also great for speech when you sell it) and you'll probably have hundreds of these dwemer ingots lygin around after a couple of trips to dwemer ruins

HEAVY/LIGHT ARMOUR: simply taking an attack whilst wearing the armour increases the skill. the more damage the armour absorbs, the higher the increase, but you cant really control that

BLOCK is pretty much the same - except you have to be holding block when they enemy attacks. Shield bash also increases block

ONE/TWO HANDED: using weapon increases skill - more damage inflicted=more skill increase

hope that helps - i found it really annoying when i started playing that i couldnt find anywhere that told me this!