I'd like to improve my ability to concentrate. But how?

First: The blood-type idea is BS. So forget about that.
Second: This has little to do with concentration, more to do with confirmation bias - you have written something, proofread it, and reread it a few times - then you convince yourself that the content is perfect, and you read it again - and it is. But it isn't. You've just convinced yourself that it is, and then your brain will cover small mistakes that isn't glaringly obvious.
Third: as others have suggested, letting it rest for a bit, say half an hour, before sending/posting might help, because then your brain "resets" between writing and reading, and it will be easier to spot errors.

While I don't really suffer this problem, I do spot some errors sometimes - I usually don't stress it, because I don't make that many of them, hence it's not really that big a deal.

Depending on your predisposition, you might also benefit from changing your surroundings - some people work better with distractions turned off (hence no music or talking or other distractions), while others work better with visible / audible distractions present. You might experiment a bit, and see if you perform better with different types of distraction/lack thereof.

 

meditation and a special device know as the neo neurophone developed by Dr. Patrick Flanagan does wonders. http://www.neoneurophone.com