Look up the specifications of your HDTV on the internet, or if you have the books that came with it, look through there. What you want to find is your TV's "native resolution". If this value is not 480i or 480p, you're getting upscaling lag from your TV.
What that means is that your TV can ONLY display a High Definition signal. But the wii only puts out a normal one. So when the TV sees the normal signal, it has to convert it into a High Definition one. But this conversion takes time, up to about a quarter of a second. That delay you see as lag. And there's nothing you can do to stop it if your HDTV doesn't have a native resolution of 480i or 480p.
Something else you CAN do is try to look around in the settings for a "game mode" or something similar. TV's quite commonly now do some processing on the signals they receive. They do anti-aliasing, contrast adjustment, etc... These things also take time do do and can cause lag, but are fortunately possible to turn off most of the time. Any sort of setting you can find along the lines of "game mode" or "unprocessed mode" or even if there's a way to turn them off individually will help.
PS: If your native resolution is something-p (ie: 480p, 720p, or 1080p) Then using Wii Component cables and running games in progressive scan mode will help lag as well. That's one less thing for the TV to have to process.
