Read higher level things - things which will tend to have a formal register. Academic stuff, etc. If you come across a word you don't know, anywhere, look it up and try to use it in its correct context. Use thesauruses a lot and try and find the most pompous sounding word for everything.
And it's not a matter of reading a lot, it's a matter of reading the right stuff. Make sure you know what every word you come across means.
They're all the tips that got me to where I am (being called a walking dictionary/thesaurus).