I've begun to play Invoker more since he's fun to play (I'd probs get a bite to eat with him also). However, it takes some time to learn the combinations for his spells. I believe I will learn them all in time, but is there an easier, faster way to learn them (maybe a mnemonic method or something along those lines)?
Play this until you get your time down to 10 seconds or so: http://dl.dropbox.com/u/75495922/InvokerTrainer.htm
Then play a bot game or two until you're confident with your hero and your build, and you should be fine jumping into a real game at your skill level. The hero is definitely hard to play, but he's not nearly as hard as people pretend he is because he's so damned good at everything.
The skillbuild most commonly used right now:
Q first for regen, R for Cold Snap, then another Q should give you plenty of regen in lane. Then you level E four times (getting Invoke when possible), followed by Q twice more. This puts you at 4-0-4-2, which allows you to summon double Forge Spirits as early as possible. This combined with Cold Snap is absolutely devastating at low levels- you will melt through armor and with three Exort orbs up you'll hit tremendously hard. From there you get a point in Wex to open up the rest of your spellbook (notably Ghost Walk). Typically you max E first, then W, to get the most casting power.
The typical item build:
If you're interested in learning basics about the hero, I did a relatively popular walkthrough a few months back that goes over the old Quas-Wex build that was popular at the time. We didn't touch much on the Quas-Exort build that I've described above, though, and that QE style is much more popular recently due to a buff in the most recent balance patch. Video:
There is a flash game where you can practice and learn faster the combinations. The game is "Invoker" and can be found at http://www.invokergame.com/
Practice is always a great way to get the feeling of the gameplay