It's simple really, you don't use just a single line to get it thinner at the end. Granted, you could use the brushes available on Illustrator, but they aren't exactly smooth. Try using a line of about 0.25pt in thickness (more/less, depending on you, because in the end it's going to have a black fill, (
Pic1)) and trace the lines you want. Instead of how normally you would vector the lines, just try to vector the outer and inner lines, and they will meet at an end point which will of course make the lines thinner at the end, (
Pic2). (I keep thinking of something to say but I can't... hopefully some pics will make understanding better)
Pic1
Pic2
I hope this helps! I seriously suck at explaining things... so, well, if you need anything else hit me up on MSN. I could give you the original .svg files if you want.