Using the correctly sized spool cap on your thread spool pin gives best results. Replacement spoolcaps and spool pins are available from Sewingpartsonline.com 

Search by your machine manufacturer in the parts section.

In a pinch, you can use a 1/4" slice of synthetic wine cork and use   an icepick or dremel tool to make the spool-pin hole in the middle. We can usually get about 5 or 6 slices from one synthetic cork.

I carry them in my sewing bag for students who lose theirs and show up to class without it.

Synthetic wine corks work well - real corks do not - they crumble.