It goes without saying..... check out vimgolf if you haven't - whether novice or expert, competing or just reading, you learn new stuff.
Don't become too plugin dependent too early, but a quick browse through some other's dotfile repositories (yadr is worth looking at, especially if you are on a mac - not necessarily to use, but definitely to see what's possible).
Things like EasyMotion really make you think.
As do things like persistent undo files. (gundo and other stuff)
At some point you should be buildilng up your own set of dotfiles from scratch.... not just relying on someone elses (some day some bug hits you and you have no idea where to start otherwise)