I soak my brushes in fabric softener, usually does the job if the product on the brush is water based.
If you have dried mod podge stuck on your paint brushes from previous mod podge projects, you can remove it by soaking the brushes in hot vinegar for few minutes, then using a paper towel to wipe them clean. You may also want to run them under warm water to dilute the vinegar smell.
I use fabric conditioner also.
Hi Joshalin, you can soak them in a mixture of hot water and fabric softener or you can use those dried mod podge paintbrushes and turn them into really unusual hooks - https://acraftymix.com/blog/2018/02/05/hooked-repurposed-paint-brush/