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I used to use a paper clip, I liked it better than a button because you can loop it into your line anywhere without having to slide it up and down the line or thread it in. There's a little trick to looping it in but pretty simple. Now I mostly just tie a slip not in my line for a marker. It can slip out when you catch a fish but I got sick of carrying paper clips around.
Used to use slip knot but 2 winters ago changed. Now use those draw string adjusters (like you have on parka strings)that are springoaded. You press on them and they open, allowing you to pass your line thru. When you release them, the pressure holds them in place. Got them at Walmart in sewing area. About 1/2 - 5/8 inch dia. roughly. Also you can squeeze them to adjust the line depth without removing your mitts. Comes in handy when its really cold.(which is often here in Manitoba)