I've pretty much given up donating to charity shops too as I'm doubtful that things don't just end up in landfill. That's not to criticise the shops - they have limited resources and staff. But unless I have something that will very clearly make them decent money, I don't bother.

Instead I either give things away or make sure they are properly recycled. It takes more effort that shoving things in a charity bag, but I think it's more environmentally sound in the long run. It also makes me think carefully about what I buy. And I support charities with cash instead.