Also, not that it's foolproof by ANY means, but consider this:
It's marginally harder to mess with POST'd data versus data retrieved by GET. Most newbies don't even know how to do it.
In general, you should always use POST to submit data, unless you're talking about something small you'd rather put in links, like a pageid, etc. One such reason would be the one I mentioned above, but another excellent point was made by TheDefender in regards to the size limitation imposed by GET.