Lack of hip and ankle mobility is a common issue among most "Westerners," because we sit all the time.
One of the best ways to improve hip and ankle mobility is the Third World Squat. Just do it for as long as you can, as many times during the day as you can. This can be as static or as dynamic as you want.
Mark Sisson, over at Mark's Daily Apple, has a number of exercises that can help improve hip mobility. His list includes:
- Box jumps
- Mountain climbers
- Foam rolling
Additionally, I'd also add in a few Yoga poses, including:
- Lotus (start with half, work up to full)
- Pigeon (this is great one to stretch that front section where the leg meets the hip)
- Warrior (all of the Warrior poses, strengthens the hip flexors, which helps support hip mobility)
About.com has a more extensive list of poses for the hips, as well.
(A note on Yoga - Yoga poses don't have to be static. In fact, most practitioners will do sequences. If you put them together right, you can flow from one pose to another, keeping you moving for the most part.)
As for your ankles, I'd look into not only ankle exercises, but also hamstring work, too.
Again, about.com has a great list in Yoga department. I particularly recommend the Seated Wide Legged Straddle and associated bends. To do it right requires you to adjust your hips correctly, and keeping your feet pointed up puts a good stretch on everything in the legs, which will help give you more range of motion, as well as strengthening the muscles in the feet (make sure you don't slack!). These will help keep your legs straight in the V-sits.
Again, Mark's Daily Apple has some other good ankle-specific work, including "foam rolling" with a tennis ball, and dorsiflexion exercises.
I'd also suggest calf raises on a slightly elevated surface. That way, you can move your heels below the balls of your feet. It doesn't need to be fast, or weighted, the idea is to work the ankles.
You can also roll your ankles around while sitting. Keep the balls of your feet on the ground, and make vertical circles with your heels. This will help get blood flowing to them and work the muscles.
Another thing to do while sitting is to plant your heels and try to lift your toes and the balls of your feet as high as you can. With the help of the calf raises, you should see improvement here after a while.
Finally, squats, squats, and more squats. Doing good-form squats, themselves will help with mobility. They don't need to be weighted or one-legged (again, because you're working on form and mobility, here, not so much major-muscle strength), just make sure your form is as good as you can get it.
For the long run, you'll want to do these things (or things like them) all the time, especially if you have a desk job. Mobility and flexibility, like strength, are use-it-or-lose-it. To get improvements quickly, it might be worthwhile to take a couple of weeks off of your normal progression routine (if you have one), and work on mobility. You don't have to stop your usual stuff, just don't push for gains, go back to the basics and focus on mobility and form. Basically, treat it like you might a weak muscle - make a concerted effort to focus on it for a while, until it catches up with the rest.