I just use acrylic over acrylic. Sometimes, depending on the colors I've used (Specifically flat black) I'll put a coat of Future over the base color before the detailing.
I use a small paint brush for molded hoses and so forth. I think it's a 20/0 "Spotter", the important thing being that it comes to a very fine point. Just be careful and there is usually plenty of room to paint them properly. If you get a little bit off, just touch it up with the base color.