There are various things that can lead to this.
i've had some trouble with it as well, but now it's solved.
i used to disconnect my vga cable, and then connect it again. mostly, that kept my screen sharp for a day or two.
but since my screen broke (it turned pink) i returned it to the story i bought it, and they shipped it to an acer center. there i got new software installed on my screen, and since then it's been sharp.
there's a note on this in the manual you got with your screen, maybe that could help you too.
Delete the plug and play monitor driver and reboot. Trouble is this worked for only one day and had to be done the next. Wish I knew a permanent solution. It appears to be a driver issue. Will see if I can reload the monitor specific drivers instead of plug and play.
Usually answered in minutes!