What model is it?? There are so many LCD's around...
Some tips:
-Try turning every image-enhancing option off. They are the source of most lag (some TV's have a game mode that do this for you).
-Set your PS3/360 in the native resolution of y'r TV, so your TV doesn't have to up- or downscale the signal.
-If you have a VGA input on your TV, use it! Most of the time they bypass all image enhancing stuff on your TV. Giving a direct "console to TV" signal, again, set your console output to the same res as your TV!