Setting marks for your actors is one of the easiest ways to ensure that everyone is on the same page. It helps make sure that not only is the blocking correct, but that the camera team knows exactly what's going on and they can get proper focus. There are many different ways to actually set these marks for the actors once blocking is complete, and in the video below, Jared Abrams from Wide Open Camera takes a look at some of those used most often.
There is a little bit of NSFW language (for those that need to know):
As he says above, really anything can be used as a mark, especially if the blocking is complicated, and it would help the actor to actually have something they can feel. I've used everything from sandbags, to wallets, to phones, and sharpie markers, so as long as the mark isn't moving, it should do its job. The best way to do it is still with tape, but there are plenty of times where you may not be able to tape the floor, or you're in a location where tape just won't stick to the ground. Also, as Jared mentions in his post, making tabs on the tape is extremely helpful, as you want to be able to remove the marks as quickly as possible.
What other materials have you used for marks before? Anything you want to add about your own process to what Jared says above?