Try learning one new vim command or feature every day, and forcing yourself to use it. If you find yourself not using it, instead doing something less efficient, undo the changes you've just made and re-do them in the newly learned way.

For example, after learning that ^R let me paste to the command-line, I sometimes would copy and paste text with the mouse. When I would catch myself doing this, I'd delete the newly pasted text and use ^R" instead.

The vim wiki and blogs like the Daily Vim helped out a lot with learning a new thing every day.

Also, the docs are excellent. Start reading them (:help toc for the table of contents).

Learn how to use regular expressions in vim efficiently! Turning on hls (:se hls) helps out a lot in learning regex, because you can interactively see what atoms are doing.

Learn how to script vim (:help vim-script-intro). Vimscript kind of sucks, but being able to automate tasks using it is great.