Also putting randomly placed, "randomly" named hidden field with "random" data works well. For instance, you could put encrypted salted timestamp in there, and

Also putting randomly placed, "randomly" named hidden field with "random" data works well. For instance, you could put encrypted salted timestamp in there, and then, when you get the POST back, make sure the timestamp is not older than X minutes. In fact, all your fields could be named that way, as long as you're willing to decrypt the names on POST.