I use MTG, whatever you do, DO NOT BRUSH! Pick with your fingers.
Some horses are naturally predisposed to thin manes and tails. Keeping the ends trimmed neatly will make it look thicker, and if she's rubbed any out, MTG can help it grow back (and can help keep the skin in good condition if she naturally has dry skin). An omega-3/biotin supplement wouldn't hurt either, I've heard good things about flax.
Bio mane! I use it and it works REALLY well