Reddit mentions of Dorman 59207 Throttle Return Spring

1. Remove stock clutch assist spring
   
2. Buy this set of springs- https://www.amazon.com/Dorman-59207-Throttle-Return-Spring/dp/B001HZKGCE
   
3. Use one of the springs from the pack as a replacement for the stock spring. You connect one end to the base of the pedal where the top of the stock spring was and the other end to a crossmember/part of the dash. This is a shitty picture I found- http://www.focusst.org/forum/attachments/focus-st-discussions/94917-does-focus-st-have-assistance-clutch-spring-forumrunner_20150430_170508.png
   
4. Enjoy your much better feeling clutch without having to worry about pedal sag

Don't do it! There's a better way! Go to Autozone. Buy this https://www.amazon.com/Dorman-59207-Throttle-Return-Spring/dp/B001HZKGCE

I used the single long skinny silver one. Hook one end around where the top of the clutch spring used to be and hook the other end on a hole in the metal bar above your clutch pedal. It barely adds to the resistance and more importantly, relieves that annoying snap right at your bite point. It feels almost the same as driving without any springs, but the clutch spring will ensure your pedal goes back up.

Only issue is you can hear a bit of the spring movement when you clutch in and out, but I got used to it after awhile.

EDIT: I see someone's already mentioned it. Oops. Anyway, the point about it being preferable to replacing the stock unit stands. You can't purchase that clutch spring without the entire OEM clutch set, and from what I've read, it's pretty easy to break that plastic piece, especially with the spring tension in the way, so I didn't play with mine at all.