Panel Lines and how to Paint Them
This is a question I get from many kitbashers/modelmakers both new and vets. Forget trying to paint them in with a brush, 9 times out of 10 you will either ruin the brush or just make a mess.
The best way I've filled in the panel lines on a mech, or anything else that I wanted them to be visable, is to use .001 tipped pen. Yes, its an ink pen.

You can find them readily at most craft and hobby stores and from time to time in the crafts department of Wal-Mart with the prices varying from $1-2 US.
Tips
These pens are not ball point nor are the felt, to be honest I'm not sure what the tip of them are made up but with it being so fine they can and will break if too much pressure is applied so be carefull.
When filling in panel lines be sure to take your time. In most cases these pens have a very steady flow of ink and wont blotch/build-up if you leave it in one spot for a few seconds. However a few of the cheaper ones have a very heavy flow which can overflow and "spill over" onto the rest of the peice your painting.
If this happens you can easily wipe them off or, if your looking to put some "weathering" on your model, use a napkin to soak up the extra ink and gently rub the remainder on the peice. In the case of mecha it usually ends up looking like a grease stain or smudged earth. The exception to this method is if the peice your painting is white. If you smudge the ink it will usually end up looking yellowed, like the butt of a smoked cigarette and not very eye appealing.
If this happens you have a few choices:
1. Soak up the excess ink with a napkin, usually the very soft "gentle" types work best but sometimes leave a little felt. No worries, you can get that off later.
2. If its an airbrushed peice, just wait for everything to dry and do a quick touch up. In most cases its usually just a very small splotch and easy to cover up.
3. If it becomed yellowed and dried, you can try to use paint thinner but in my experiance, its better to just repaint the entire peice.
It does take a bit of practice and a steady hand to use these pens but in the end its worth it. If I get my hands on a digital camera I'll take some pics of what I've painted with them.
EDIT
I forgot to mention that this method can be used on action figures as well. The key to getting it to work is cleaning the figure. Virtually all plastic figures from mechs to humanoids have some residual grease on them from the mold that can and will mess up any type of paint job you attempt.
I just put the figure in a large bowl of warm water with a few drops of liquid Dawn and let it sit overnight. Now all the grease should be off in the morning when your ready to fill in those lines. I slowly pour out the water so the grease doesnt get back on the figure then put it under a running tap of hot/warm water for a rinse for about a minute. Allow time to dry and begin filling in those lines.
For those figures that have stickers on them, use your best judement. I would recomend using a very soft bristled toothbrush to clean away the areas you plan on filling in with a small amount of dish soap on it so it doesnt sud covered in suds. It shouldnt take too long to clean off and dry, maybe 10 minutes at most.