Will you have a completely empty stage, or will the lighting designer have booms for sidelight? If so, that's a great way for them to visually align themselves.
If you have a truly empty stage, colored cue/xmas tree lights on offstage walls can help them position themselves, if sightlines & design allow. Masked LED's on the downstage edge of the stage are great - center line is one color, quarter marks a different color. That gives you a visual grid that only the performers can see.
You can put small LED lights on the upstage sides of scenery for visibility & safety.
I like shape symbols over colors any day, for floor marking.
If you find you have one or two performers who are very good at hitting their marks, spike their mark, make them the "anchor" and others position themselves relative to him/her.
Most importantly, make sure your stage manager has enough deck assistants to watch scenery fly and can communicate with the rail to stop if a performer is in danger! If anyone wants to cut deck positions to save money, support your stage manager and argue to keep them! Your SM and your performers will thank you.
-Colleen