What are some great drills do you guys like to do for takedown defense? What are some of your favorite single leg and double leg defenses? What are things you like to do to not get taken down.
Your 3 layers of defense should be: level change. hands (arms) then hips. the earlier you can stop them (in this order) the safer you'll be.
See if you can find a video of Iowa doing their stance and motion drills. It's a drill to work on what bull describes above 
Punish the head
Take him down first!
One thing I do... With my kids, I have one put his hands behind his back and then try to prevent or block shots with his head position.
I started wrestling late and really had to work on my takedown defense just to get through practices ok; I improved a lot doing king of the hill (you wrestle for takedowns and the winner stays in an the loser switches out with the next guy), which also helps you get better at the takedowns themselves (which IMO is also helpful for takedown defense), and doing intervals with sprawls on my off days. I did them as fast as I possibly could up and down for months, it helped me out a ton. Even if I never managed to develop a decent shot I built up a fantastic sprawl. The better the wrestlers you can find to shoot in on you the better your takedown defense will be.
There's more to takedowns than just leg attacks, though; if you want to thorough and can afford the time investment you could try and find some good Greco or judo guys to drill with, as well.