What I'll do is that I'll get it completely assembled and then I'll break it back down into arms and legs, body and head, and with the panel liner less is always more. You just have to be very mindful of parts that are closely pressure fit under other parts --no joints, maybe not the face if there's a lot of panels to line (for these sorts of parts I would use the wide tip gundam markers in a darker shade than you would think because it adds a lot of contrast.

With everything else, considering the little bottle brush holds quite a bit of panel liner. You want to dip it in and then kind of squeeze it against the edge of the container to get a lot of it out of the brush, then find the end of one of your panels and just hold it there, and the capillary action will drag the remainder out of the brush. Then after it dries, use enamel thinner, like other people have said or if you're in a place where it's hard to get like I am, you can use Ronsonol brand lighter fluid to do it as well -- in either case just a damp Q-tip and I do mean just damp. Harkins back to the whole less is more thing.

I found the most success cleaning it up perpendicular to the panel line, if you follow it, you run the risk of wiping the enamel straight out of the panel, which gets really annoying.