"Blind spots" or "Throwing away rating points".

I was hoping to get a few helpful comments from this post.

A friend of mine challenged me to a game over the board a few days ago; he's normally a 600 newbie.  I was doing well - with a nice material advantage - and had managed to calculate a decent series of sacrifices which would of resulted in a win; alas, I didn't his Queen.  By the end of the exchange I was down two Rooks and a Queen.

I've made similar mistakes in other games, where I didn't see a particular piece - or even worse, forgot a piece could move (That's right Pawn's can capture my Knight). 

I was wondering if anyone had any helpful advice for fixing a weakness like this?  Or at the very least if an original joke.

Thanks in advance.