This is the first in a series of tips that I've found to help me speak and write better English over the years.  Simply put, learn another language.  Yep, that's right.  Sounds a bit off the mark, but I have found that learning other languages dramatically increases my understanding of my own language. 

In fact, learning other languages helps to learn other languages, especially when they're in the same language group (e.g., the so-called Romance langauges).  Some people say learning German is hard, but I think it is one of the easiest languages for an English-speaker to learn precisely because English has very strong Germanic roots.  When speaking with and listening to my wife's family (who are native German speakers), I can't tell you how many times I've been able to guess the right word or understand the meaning of a word simply because it is so like English.

So, if you've always wanted to pick up Spanish to watch Sabado Gigante, if you want to learn Latin to read medieval philosophy, or if you want to learn Gaelic so you can sing proper mouth music, just do it.  There are tons of self-teaching resources to learn other languages, even dying ones like Scottish Gaelic. 

Even if you never speak it fluently, or even if you don't pronounce it correctly, learning another language will give you a stronger grasp over your own.  I'd recommend Latin and German especially for this purpose, but French (thanks to the Normans) will also help a lot in this respect.  But again, any language will do because it will make you more aware of the structure and peculiarities of English.