Dandayamana Bibhaktapada Paschimothanasana is the sanskrit name for the posture we call Standing Separate Leg Stretching Pose, but there are other possible translations that come to mind: Slip Sliding Away Pose; Wishbone Pose; The Split. You know what I'm talking about... "take a BIG step to the right, four feet minimum."
One of the challenges for many beginners is that the four foot step quickly becomes four and a half feet, five feet, five and half, oh no, almost six feet apart and still sliding! Adding to the frustration is the insistence of the instructor that what you're experiencing is intentional and that it offers an opportunity to work on developing inner thigh strength (don't they realize that telling you they're called "adductor muscles" doesn't make it any easier?). Oh, right, making it easier isn't in their job description...
Obviously the main reason for the sliding feet is a lack of inner thigh strength but there's another reason and you're standing on it. Chances are you're standing on a freshly washed and dried towel (or mat, if you use the cloth variety). That's great but it's also a major cause of your struggle in this pose. The drier and fluffier your towel is the more slippery it's surface will be. You want it to be clean, of course, but if you take it out of the dryer before it's completely dry -- or better still, air dry it -- it's surface will be a little more coarse and much less slippery.
It won't stop us from driving you crazy with our "it's supposed to be hard" admonitions but you might find yourself being able to focus on other parts of this posture instead of simply struggling to keep from sliding into a full split.

Comments
thank you for this tip!
thank you for this tip! although i think i'm still going to invest in a yogitoes mat to help out with this problem. ;)
I was not born for sales.
If I had a different sensibility I guess I may have written that tip to suggest people buy a Yogitoes towel from us. It didn't even cross my mind but now that you mention it...
Thanks
Silly Slippery Towels
Rebecca R mentioned in class the other day that using a dryer sheet will make the towel more slippery. I never use dryer sheets on my towels because I've heard that it makes them less absorbent, but what Rebecca said makes a lot of sense. Just some additional advise on this subject :)