This is an area I haven't gotten stellar results in either, but they've been fair. Below are a couple of examples...
1: After matting and grouping the photo and mat together, hit Effects>Distort>2nd left option>slider set to -4. Then I used TouchUp>Other>Dodge and Burn> settings:8 and 0 stroked across the highest curve, and settings -8 and 0 stroked once or twice only in the deeper curve (large brush size). Then shadow the whole thing.
2: After matting and grouping the photo and mat together, hit Effects>Skew>drag left handle slightly upwards. Then repeat same Distort option above, but using the first left distort option, set at -11. Same Dodge & Burn and shadowing.
3: Use your marquee tool to cut out the corner you wish, Edit CUT> Edit Paste>Flip Horizontal. Now use your rotate arrow and arrow keys to nudge it a tiny bit into place. You should now Fill with white or whatever your mat color is. Then Distort Brush> Shrink> and just play with your brush size,reset and stroke placement to gt the bend where you want it. The Dodge and burn as needed, highlighting on the upper most and burning on the deeper curves. Shadow and done.
It sounds involved, but really isn't too bad once you get the hang of it. Hope this helps!!