Things I've noticed so far:

1. You can't do a constant effect unless you have a very powerful
soul gem. They're very expensive and/or difficult to come by.

2. As has been pointed out, the LOWEST strength SOUL GEM will trap
the soul. That is, the lowest for the SOUL that is present in
your inventory. If you don't have a big enough soul gem, then
you're out of luck.

Enchanting things I've noticed:

1. Enchanters NEVER FAIL, you probably will for quite a few tries.
But they are EXPENSIVE.

2. The CHARGE CAPACITY is equal to VALUE (gem)/10.

3. If you DON'T have the spell, you can't put it in an artifact.

4. Do lots of practice with the little soul gems on non-valuable
items. Once an item is enchanted, in my experience, you can't
add more later.

5. The more valuable the item, the more ENCHANTMENT that can be
placed on the item.

6. When you adjust the magnitude, note that you can usually squeak
out a little more power before you exceed the ENCHANTMENT level.

7. Enchant weapons with "Weakness" spells. Such as a weakness to
frost, combine it with an elemental Frost spell and bingo!

Odds & Sods:

1. Some combinations have funny interactions. That is, despite the
fact they DON'T have an effect (say lightning shield) listed in
their properties, you can END UP with that when you're combining
for other things.

2. Guild Ratings - If you highlight your FACTIONs (on the factions
joined) it will tell you what level(s) in what skills you need to
advance. Quite handy.

3. Custom spells are great. Buy a cheap spell and you can have it
tailored to your abilities or combined to form less-resistible
spell effects.

I like to combine three Destruction effects, like Poison, Shock
and Frost...

I also like to combine the weakness spell with the elemental
damage spells. That keeps me from having to switch in combat,
which can be difficult at times.