Yeah man you just gotta do it. If you have something you think is even remotely worth recording throw up a mic as quickly as possible(or keep one set up like others said) and hit record. Don't worry about mic placement or eq or anything. Then after you get it down, you can go back and record properly. I have had a lot of cool song parts go bye bye because I was too lazy to get them down and I forgot them. Make some rough demos and then go back and re record properly.