Always ask for what you need to know. It's a good thing to learn : )
Walking with intent (not the walk they do in the paddock) up and down hills, trot up and down hills, carefully and with balance. A couple times a week will work well. If hills are not available to you, walk anyway, and walk on, no touring or rest stops for a good 10 minutes to start. When you ride, do transitions frequently. Walk to trot to canter to halt to walk to halt to trot to walk, etc.,that sort of thing. Be creative : )
Riding correctly will help, too. Be sure to properly engage the hind quarters (feets under belly, not behind butt) if it is there you want to develop. One should ride with balance anyway. A moderate amount of protien in the diet if working harder can help too.
Not sure what 'dimple' you speak of... it might just be the horse's normal conformation or current level of conditioning. Low water intake will hollow the flanks...
Hope this helps!
Quick aside: Heyycutter, I am from MA too! Howdy : )
Not creepy for me, I dare anyone to enter here, lol : )
I am in South Central. PM the rest if you like, as in where you are out of..would love to chat : )
And on dimples, if above the rump along each side of the spine, it can be well muscled, or OVERweight.
Again, if hills are not available: and deeper footing is, work in deeper footing but be most aware of the workload you are asking. Too soft footing can be quickly and easily hazardous to tendons when unfit or casually used.
(gotta be have, new here. Sorry for deviating from the OP)