I think my biggest problem is trying to have all the spells too soon.
I bought a bunch of scrolls and maxed out my UMD each level, and took the wand/scroll enhancements so I could use scrolls/wands. I would buy up every scroll I could find and I got into using scrolls even when I only had a 75% chance of success and then I would end up not casting something because I failed the dice roll. I was always late in helping my group. I ended up with four hotbars full of scrolls that when used caused me to lose on of my scepters (I have a potency and a lore scepter set for ice and one for fire) and I would forget to put them back and lose damage on my next cast.

I think I should just use real spells in my hotbar because they can be casted faster, but my problem was always which spells. I have learned the hard way, by having many deaths, to look at the quest first and decide what to put in the hotbar, then change them at shrines if my first choices were wrong. By picking spells which statistically have more effect, and cast faster, I have become a greater help to the groups I am in.

My WF Wiz is only mid level and he is fire/ice specd with Heighten feat. I find that Niac's, Web, Haste, Hypno and the Resists have been the most useful spells, and I always keep pots/wands for self healing.

I am learning all the time but I just wanted to post my mistakes for other noob wizards to see.