Mad Alchemy Embrocation is what I'm using now- they have a lot of different formulas for all kinds of conditions (and nice wintery smells, like the Russich Thee). www.madalchemy.com - get it directly from them.
They're a small company outta MA that supports 'cross. Definitely check their stuff out.
I started using stuff from Northwest Kneewarmers this fall and I love it. It comes in Mild and medium. Rumors abound that a "Hot" version will be released soon.
Formulated by a naturopath in Portland, Oregon. It's designed to wash off with soap and water (though you need to wash more than once and scrub a bit -- it WILL stay on in a wet race).
I tried the Mild at a practice and found it lacking in temps below 50 degrees, so I used the Medium at my last race and found that it was great.
I put Mad Alchemy 'warm' through the paces it Wissahickon. It did a good job keeping my legs warm under the mud, it smells good, and it isn't very expensive. I just ordered the Hot from my LBS for the winter.
I use Mad Alchemy and can definitely recommend it. Get at least medium and possibly hot as well. I've got some mild and it doesn't do much except when there's a slight chill, or a way to keep mud off you when its warm.
I've tried some other brands like Record and Born but now I exclusively use Mad Alchemy. I live in Michigan so we have some cold 'cross races. The medium warmth seems to be the perfect temp. I have both the regular medium and the Russich Thee (this is my personal favorite).
I got it online directly from Mad Alchemy - they are a great company, easy to work with and responded quickly to any questions I had (how do you get this stuff off? - dry towel and baby wipes was the response - seems to work well)
There is a mental component too. I am not sort of softening up to say just avoid using it when there is a risk of hypothermia. In those cases a thick heavy mild formula would be warmer (for your core body temp) than the medium or hot.
I'll tell yall a little secret...
(i've been using the Leg Salsa stuff this fall - and I looooove the post race burn it has. Doesn't seem to do squat pre-during the race that I can tell, but that post race buuuuuurn feels awesome - oh and it makes my skinny white chicken legs look slightly tan!)