What started my obsession with making bows was when I bought my first 'no slippy hair clippy'. When I started making them I looked for the magnets too. Honestly, the magnet does nothing. What makes the difference is the velvet. If you fully line a 2 prong clip it will stay in the hair NO PROBLEM, but its rather bulky on small heads with short hair.
After some trial and error, I now partially line a single prong clip with 4.25" of 1/4" velvet ribbon. You can find the velvet at The Ribbon Retreat. Its $15 for 30 yards.. so I think it works out to you being able to make 250 clips, or something like that. But you have to purchase it on the wholesale site. I think the partial lining works great. I would put the grosgrain partial (and fully lined) clip in my daughter's hair and it would come out, but with the velvet it stays great, and its not as bulky.
If you want to fully line a 2 prong clip, then I use 6 7/8 " of 3/8" velvet.
Hope that helps.