Get your a friend to sit on top of you it will cover you up, and if its a chick sittin on you, the attention will be taken away from the site of your revealed undies, and will be focussed on the fact that there is a girl sitting on another girls lap!
If you feel uncomfortable crossing your legs, or squeezing your legs tight, then bring a little jacket to through over your legs, so you can sit relaxed.
Depends on how low the sofa is and how short the skirt is! The shorter the skirt and the lower the sofa the more the skirt rides up. So the answer to the question is: with difficulty! When trying on a miniskirt, girls should always try and sit low.
that one cannot answer as if i would be wearing mini skirt they would think I'm gay but for a man i did think your question was interesting to say the least.