I don't know why people have so many problems using the mag bars. I've lit dozens of fires with them and never had much of a problem.

First off, I do use a piece of old hacksaw blade (the cheaper the better, the bimetal blades don't work as well as HS steel) with the teeth ground off. I scrape the shavings into a dried leaf or a wad of fine textured dried grass. You can even take bigger dead grass and rub it over a stick, or just beat it with a stick or rock. It'll break down the fibers so you can get a wad of wispy, hair like grass fibers. I don't scrape off a lot of magnesium, just enough to get a dusting maybe an inch square. Don't even really care if it's in a pile or not, just so it's caught up in the grass.

The best way to scrape the striker is to have the bar braced to the ground or on a rock at about a 45 degree angle with the wad of tinder under the corner that's lifted. When you strike the flint, put the hacksaw scaper firmly on the flint rod and scrape down firmly like you were trying to gouge it out. I've seen people doing a fast freehand scrape and they will only get a light shower of sparks. If you bear down firmly, more like you did to scrape the magnesium shavings, you'll get a heavy shower of hot sparks. If your tinder is really dry, you don't even need the magnesium.

The other method is to brace your hand holding the hacksaw scraper on the ground and pull the magnesium bar up. That works fine too, but I think the real trick is to do it firmly, and not the wild hitting striking at the flint thing. A little puff on the wad of grass and you'll have it flaming, ready to add to the tender.