If you’re interested in a girl’s advice (and one that is just learning herself)…
Tumbleweed had it pretty good.
My first time out, it was easy trails and an easy pace with a little spice (intermediates) thrown in just to show me some excitement. And I went SUPER SLOW the whole time. Not because I couldn’t go faster, but more because I just wasn’t sure what to expect and my safety/panic mode sensor was in partial alarm at all times. The boys led the way, and they’d get up ahead a little bit, but never so far that I couldn’t hear them at the very least, and usually see them. When I got too far behind cuz I got stuck on something, someone would come back for me. I worried about sucking so bad, and being so slow, that they weren’t having fun. Eventually, I started to realize that part of what made it fun for them was that they were getting to take me out on the trails for the first time and pass on the awesomeness that is mountain biking. So get that point across, if its true for you, because that took a lot of the pressure off. Oh, and help us celebrate the little victories! Because sometimes, it seriously feels amazing to make it over a root or up a climb, no matter how small and insignificant it might seem to you 
Don’t make fun of us when we wimp out. Give us the heads up when we should wimp out. But encourage us to stretch ourselves, especially when you’re pretty sure we can do it, cuz then we feel like a million bucks.
Be understanding when we’re trying and having a hard time. Usually I don’t like to admit I’m really struggling even if I am, but the boys I go with take care of me and call for a break at just the right time. Its super nice, because that way I don’t feel even more like I don’t belong and I can’t keep up.
Make it clear that it isn’t about being a rock star. Its about having fun and giving it a try. I’m not afraid to put my feet down. I’m totally ok with walking. And I’ve gotten the falling thing down pretty good. I go home dirty, tired, and sore, and usually with a little blood, but always with a REALLY BIG SMILE. And thats what its all about. Well, that and the hokie pokie