Alot of Cardio can really burn off good muscle if you do too much. Even glute-focused cardio like ellipticals can deteriorate a good cheek, rather than build or tone it. Yoga can be very beneficial to your "heiney", but it depends on what kind your doing. More of a strenuous flow can do wonders to help firm, strengthen, and stretch the buns, but a gentle flow where little burn is felt and little emphasis is put on the lower body does alot for your stress level, breath capacity, and flexibility, but not much for your rump.
To keep up your current rotation without anymore deflating and actually build up what you've lost, consider incorporating some sprinting into your cardio. Ever see a sprinters butt? Round and hard. Add 20 second sprints into your runs if you're a runner, and add some heavy duty uphill running - your hind-end will be on fire! Consider running bleachers at your local track for some more variation. These things will definately tone the butt while your burning fat!
As far as yoga, here is a butt-blaster combo (for seasoned yogis) I do in my yoga classes sometimes: do 3 sun salutation flows, then on the right side, do crescent pose, warrior 1, warrior 2, warrior 3, then move to half moon, revolved half moon, standing splits, up to eagle, and back to warrior 3 with a bent knee variation and then back to crescent and then down dog and switch legs...if your bum isn't cursing you at that point then you really are a warrior! Enjoy!