What is the best way to increase the mass and roundness of my butt?

I am working hard to get a firmer rounder, more massier (is that word?) butt. I am currently doing squats with the bar with a current weight of 35 pounds on the bar. I also do hamstring curls with about 40 pounds, leg extensions and the leg press machine, inner/outer thigh screen and also the butt lift machine where you lean over and put your foot on the plate and kick backwards and upwards at the same time with a weight of about 55-60 pounds. This exercise wears me out!! I hate it but I know it's a must if I want a better butt. Can anyone provide any suggestions on whether or not I'm doing the right excercises and if I should do more or something different, like maybe a hack squat? Or the reverse squat where I'm upside down and I press upwards?

go up the stairs 2 steps at a time.

You're definitly on the right track! Squats with the bar are the #1 way to get your butt into shape, I'd say.

Squats and deadlifts all the way - also, try One-Legged Deadlifts for an interesting variation that really targets your glutes.

 If you do heavy barbell squats, regular deadlifts, and the one-legged variation you can pretty much dispense with all the machine exercises ;)

Squats, deadlifts, roman chair, and lunges seemed to do the trick for me.  Not that I was looking for a bigger butt, but whatever!

Yoga Butt! I like them and Goddess Booty. I have noticed a total difference in the roundness and firmness of the booty. Plus those Goddess Booty chicks are nutty, they make me laugh.

Borrow my genes :P

you can have some of my butt!!!

doughnuts!
also..

balloons
pillows
small cats

....

seriously, what is the worlds disposition with huge round butts?

firm up the chest, round out the butt and then you complain when guys dont look you in the eyes. Seriously, pick a direction.

thank goodness for being gay ^_^