I would say reading is the best way to improve your writing. Language has a lot to do with mimicry. It's about repeated exposure to all the phrases and vocabulary people use. We naturally pick up on the expressions of others and accommodate (sociolinguistics term) to match their style. So if you want to write better blogs, read the blogs of authors whose style you admire. Read newspapers and magazines. Read books on the subjects you write about. Reading old books will only help if that is the style you are going for. Language changes over time, there is no absolute right or wrong way to write, at least not for English.
If you do want to be anal about your grammar and punctuation you could always read up on something like Strunk and White's Elements of Style: http://www.amazon.com/Elements-Style-Fourth-William-Strunk/dp/020530902X. It's quite old and pretty crazy from what I hear, but some people swear by it. There are other prescriptive grammar resources out there I'm sure.
If there are certain silly mistakes you keep accidentally making maybe there are some language games out there that could cure you of your bad habits.