What works for me is have a handy "ready to donate" box and when I come across things I don't ever see myself using into the box they go. If I have duplicates, I force myself to hold onto only the best. Example: "red, white & blue" patriotic themed items. Sure it's great to have some in your stash but how many 4th of July pages will I ever make? So instead of keeping the 25 odd items I had, I narrowed it down to the best 5.
Then about every 6 months or so I pack up my donate box and actually donate it. I have NEVER regretting giving away anything.