You're right to be concerned about communicating the need to hold. In playing and watching other people play video games, I've often seen users (players) conclude that some action doesn't do anything (or is broken or has some unmet requirement) because they did not press and hold a button, or in some other way did not “hold still” long enough to see the effect.
I'm not aware of any widespread convention for buttons that require press-and-hold. However, continuous actions very often come in pairs — “up” and “down”, “in” and “out”, etc. My first suggestion is make pairs of movement actions clearly pairs.
This could be made skeumorphic by giving it the look of a rocker switch which sticks out of the surface on the ends, and can be pressed in either direction but not both. This can aid understanding by communicating a familiar physical interface, but carefully weigh the added visual clutter and reduced contrast of using graphics imitating physical shapes.
If you have four directions of movement, make it look like a “directional pad”, a very familiar interface for movement control.
If you have both paired and unpaired press-and-hold actions, make sure the unpaired hold buttons are more visually similar to the paired ones than to buttons that are only tapped.
Second, if it does not conflict with safety or other functional requirements, make sure the effect of the button is responsive even to short taps. If the user taps the “up” button and then sees and hears the machine go up very briefly, then they will quickly realize that they should try holding the button. Focusing on responsive control will also help the user operate the machine more easily, pleasantly, and accurately.
If the effects are not obvious to the senses, or it's not appropriate to allow short actions, then add an explicit cue: When the user taps the button quickly, pop up a “tooltip” over the button with a message like “Press and hold to move.” Make sure that this popup is visible given users' typical hand placement, and does not obstruct normal use of the interface (e.g. does not cover other buttons), in case it falsely triggers.