As a 34 druid right now, I almost never used divination. Now it's coming back to haunt me. To cast sup. camo, now it costs me at least 10 fizzles. My skill in divination is only at 12. I think all spell casters get true north at lvl 1, so if you are waiting for some guy to spawn or are bored, then cast away at true north (it only costs 1 gold if you don't have it already, and if you are really lost, and don't want to use loc, then you can use this to point north). You'll be thanking yourself in the end.
I understand that casting spells over and over will increase a skill level(eg. casting a direct damage spell over & over will increase you Evocation skill lvl).
Is it the frequency that an evocation spell is cast, the amount of mana used, or something else that determines when the evocation skill increases? For example, being a 44 Necro my evocation was around 115. @ 44 I got a very nice DD spell but it kept fizzling ( due to low evocation I believe). Am I better off using a low mana evocation spell many times or a high mana evocation spell a few times to raise my evocation skill level?
Spell skills don't seem to have "trivals" like trade skills. I can cast "True North" and still raise my Divination skill, for example. In my experience, the best way to raise a particular spell skill is to pick a low level spell that uses that skill and cast it over and over and over again. Raising your INT or WIS will help you skill up too, but the level or mana useage of the spell doesn't seem to matter.
Some spells do have higher fizzle rates than others, though.
The different 'modes' of spells you refer to are the different types of spells governed each of the skills you refer to. Each spell has a type, the following are what fall beneath each heading:
Abjuration - Most buffing falls under this. Skin like Wood is an example relevent to yourself. Alteration - Healing Spells, Evacs, and many other 'miscellaneous' spells that change something, such as illusions. Minor Healing is an example. Conjuration - The act of conjuring objects, anything from pets to some DoT's to food and water. Summon: Water is an example. Divination - What I like to think of as a miscellaneous category. Things such as see invisible, Invisiblity and Invisibility versus Undead fall under this category. Evocation - The last of the five spell categories, this one is generally associated with damage-dealing spells. Burst of Flame is an example.
Now for your two others:
Riposte - A melee class skill, generally the last obtained, that gives melee the opportunity to block a blow and return it immediately, dealing damage to the enemy while preventing it to oneself. Channeling - A caster skill, obtained by INT casters at level 1 and WIS casters at level 4. Channeling can be raised by ALL spell casts, regardless of the spell's type. A high channeling skill will help prevent interrupts from being hit in melee combat. For example, a high level caster can still get a spell off after being hit many times in the casting process.
i find this to be the hardest spell skill to keep maxxed out. it takes a lot of casting of low level div spells to keep it on par with my other skills .I know its not the most exciting thing on earth to cast true north a billion times ,but i make sure to do so. I know I wouldnt want my invis to drop in a dangerous area and fizzle all of my mana away.