Just posted this in another post but thought I'd add it to the sticky:
Thoughts on Yoga being Difficult
For a lot of you that are having trouble with yoga, make sure you do only nose breathing (deep, slow, and fluid), and only go as far to where you feel something, but don't feel so much that you'll crash if you keep working at it. It should feel a bit like you're struggling, but as you breath through your nose deeply and calmly, that struggle should dissolve. If you can't do this in your final "place" in the pose, you're going too far and/or pushing yourself too hard.
Length of Yoga X and Scheduling
To be fair, Yoga X is a bit too long if you're working out an hour each day already as it is, but I've actually modified the schedule by switching Kenpo X and yoga so that yoga falls on the end of the week, which is quite nice in that I can still get a good stretch on Saturday and a rest day on Sunday, while keeping an hour a day on the weekdays. I highly recommend it, though yoga will be more tough after having done legs and back the day before. Sometimes I do Yoga X, sometimes I do another video I have.
Yoga X Alternative(s)
Speaking of other videos, for those of you who don't want to put in an hour and a half for yoga -- which is admittedly quite a lot but pretty typical for a studio session -- I highly recommend Bryan Kest's Power Yoga instead, which is a 50 minute exercise:
http://www.amazon.com/Bryan-Kest-Power-Complete-Collection/dp/B0001WTWYC
Some other people recommend "Fountain of Youth" from One on One with Tony Horton. While shorter, I've watched this video and found it suffers from the same problem that Yoga X does, which I'll talk about next.
Also, many frequent yoga practitioners I know have had very positive things to say about Baron Baptiste and Rodney Yee, though I haven't checked out any of their videos personally. I did frequently go to Baptiste's studio in Cambridge in my undergrad years, and found the sessions to be absolutely butt-kicking.
Yoga X v. Bryan Kest
Tony does go through a great sequence of poses in Yoga X, but he gives literally NO instruction on how to breath properly (which is perhaps the most important thing in yoga), as well as scant info on how to properly approach sensation in the poses; finally, though it's not everyone's thing, Tony doesn't really make much of the more metaphorical/spiritual side of yoga, which I think is actually a pretty cool thing that can be ignored if you're uninterested and taken advantage of if your mind is open to it.
In terms of difficulty, the Kest videos have 3 levels of increasing difficulty (each 50 minutes), so you'll progress to the difficult poses as you advance through the levels, without having to waste your time getting frustrated with not being able to do something in Yoga X. Kest's instruction is terrific, but on the down side, if you already know what you're doing, he probably talks too much and that can get annoying. Also, aesthetically the video isn't as pleasing given the funky outfits people are wearing (it's from the 90's). Nevertheless, for 8 bucks, it's in my opinion a must-have if you're looking for an alternative to Yoga X.