madtripper,
Funny you should ask that b/c I just had a berm built that is 12" graded down to about 6 " in the front. It is backed with a retaining wall--in the back of my garden under huge trees. (I'll have bright shade). I did this b/c this area tends to flood (& freeze in the winter thaw)& I needed to raise my perennials/shrubs' roots. I've only just built the berm. I haven't planted yet. I have, however, looked into a lot of shade/part shade perennials. Did you need some comments about that? I'm afraid, though, that this berm-- which I did line with foam board for insulation by the way--hasn't gone through any seasons yet.

Over the years, I have constructed raised beds in my woodland rear garden, that vary in height from 12" to 30". You must be careful to observe the root zone of existing trees and not cover them with more than a few inches of additional soil. The elevated beds will also need weep holes at the base for drainage to relieve the hydrostatic pressure exerted by stored water.
Have seen elevated flower beds at some public gardens, constructed for the enjoyment of all, but especially those in wheelchairs.
I have mostly small plants at the front of the beds and the tall one's at the rear. There is a mixture of native and non-natives, for year around interest. After many nights of frost and freezes, the deciduous shrubs and herbaceous perennials have hibernated for the winter, but the evergreen Asarum, Heuchera, Goodyera, Helleborus and some ferns will provide winter interest. Although my property is a designated Wildlife habitat, I avoided planting berried plants and grasses in these beds to limit the maintenance requirements. Advanced age has it's limitations. I'm afraid!
Good luck and enjoy your efforts, it's well worth the time and energy!
RB

Like the previous poster, most of my shade garden is bermed. It's a pretty good portion of my urban backyard, about 1500sf, and has several paths with stone risers wandering through it. This allows planting of those small and delicate little woodland species up close and personal. IMO, a bermed or naturally mounded raised planting area is more suited to the character of a shade garden than a formally constructed raised bed, but your specific conditions may require the latter. And that is an excellent point about taking consideration of existing trees and avoiding piling soil over their root zone. That's a sure-fire way of killing off your primary source of shade!