0000 steel wool, your not gonna have the shine of new plastic, but it will take some off and brighten them back up.
you can also use mop & glow, this will put like a wax coating on them and give you a nice little shine. the finer stuff you use the better they will look. just dont go crazy and start sanding them with 80 grit. you can also wet sand them. just use very fine material. they will always be brittle tho if bike sat outside most of its life.