I've been playing for a year, very sloppily, and I have started to pay real attention to my left hand muting. It sucks.
Are there any good exercises/stretches which would help?
I know what to do, but I have a feeling I'm doing it wrong.
Cheers.

Thats a tough thing to practice. For most it will come naturally after a lot of practice and you have found techniques you are comfortable with. For instance, I like to use my left index finger to mute the higher strings and the palm of my right hand to mute out the lower strings, whereas a friend of mine prefers to use solely his right palm and hardly does any muting with his left hand. Hope this helps.

Muting in general is one of those things that comes from practise. If I consciously think about my muting technique it will go to hell every time.

Fret hand muting is definitely something that takes practice.

Pick hand muting (palm muting, whatever you want to call it) is definitely an easier technique to master and obviously more widely used.