THE CAPE:
Fill a small bowl with warm water, add a good squeeze of black acrylic paint. Mix it well, until the contents turn into a pretty dark mess. All the acrylic will not mix into the water evenly. thats good. Dip your cape in, squelch it around with a fork or something, then take it out and look for the black dots of pure paint which are stuck to the cloak. Rub them into the damp cloth with your fingers until they become black stains (if you don't like the particular placement of a spot of paint, run it under cold water quickly. Once you have dirty wet cape, lie it down (plate, draining board, whatever) Then apply paint to the bottom edges of the cloak to simulate it has been trailing o the ground. Paint rub, paint rub. Use a flat wide brush for best results.
Now, take your wet cape and wring it, twist and twist into a knot. This will form crinkles and folds and add texture and detail. When it is as knotted as possible, put it on a radiator or somewhere hot to dry.
Then put on grievous, It will be a bit stiffer, so you can pose it a bit better. Wetting it on Grievous will let it hang better, but its trial and error.
Final thing is to add screen specific dirty spots (have a look at some screen grabs) you can do this last with a black wash as described above, do it on grievous though.
Good luck with it all...