Question
I live in southwest Virginia. My yard is in 2 levels. Where the 2 come together is a rather steep bank about 10' high.  I need a ground cover to keep out weeds.  Have some lilies near the top and a few yucca here and there.  Other than that, just weeds.  Please help.   The bank wraps around and the part farthest from the house is slightly wooded and has rocks.  There are 2 large trees on upper level that also shade.  

Answer
When I lived in VA, I was constantly fighting the weeds (invasive grasses).  Even though it has a relatively harsh climate (winter cold & summer heat), some weeds just love it in Virginia!

Since you're probably in the Tidewater area, your zone is a little less severe - my guess is a USDA 8.  Thankfully, you have lots of choices.

There is the standard "Creeping Juniper", but this takes time to grow in, but certainly worth the effort in the long-run.  

Juniperus horizontalis cultivars like 'Blue Chip', or 'Price of Wales' will get very dense, and can spread up to 6 feet, but it will take a while - or you can just plant them closer together (more cost).  

Other tight, spreading Junipers are the J. sabina - 'Broadmore', 'Buffalo', or 'Skandia' varieties - it really depends on what the local nursery can get for you.

Some other groundcover options might include the Rockspray Cotoneaster, or Ivy.  

You want to plant something that is just as "mean" as the weeds your trying to choke out.  Most all of these suggestions can grow in part-shade, and in a zone 8.  

Here in Nevada/California, there is a great bank plant, called the California Fuschia, but I doubt it is available in VA.  

If you line the "grassy" look, you might also think about a nice, tight patch of Liriope, but it is a bit over-used in Virginia.

There are many more choices, but other than planting some invasive blackberries (which I LOVE!!), I think this will get you going.  Your local nursery may also have some suggestions.

There is a lot of hard work ahead.  You must remove the existing weeds and their roots, first, or they will just grow right up through anything you plant on top of them.  

Finally, whatever you end up planting, you had better enjoy, because these plants crowd out others - they don't "play nice" - which is the reason you want them on this rocky slope, right?  

The best of luck to you! -Marc