My parents donate stuff to Salvation Army. Mom makes a list of everything going (3 t shirts, 2 pair pants, etc), then assigns thrift store values to each, does the math and gets a total. That's the amount deducted. If the total is over whatever amount, then an additional form is filled out for taxes. The list and receipt are stored with the tax documents until I clean it out. (I made them shred/toss 30+ years of tax documents this summer. They still have a box of the more recent stuff, mom was freaking out so we kept 10 years instead of the required 7.) Never had a problem with it, and it's been done this way for 30 years.
If you're donating bigger things, other rules may apply. But clothes, household goods, etc this should work.