We try to do it about once a quarter, but that's because Purple Heart calls to let us know when there'll be a pick-up in our area, and telling them "yes," makes me commit to providing something
. (We take picts of what we donate and date them before they go into the Purple Heart bag/box, so at tax time, we can itemize properly.) If it weren't for making that vested commitment, purging would be much harder.
I've been getting better at being realistic about which things to purge and which to hang on to (or save for my sister if she wants something). There are some things I'll never part with, like my letter jacket from H.S. or my toe shoes, but that's because there really is a fundamental personal/sentimental/historical attachment to them. The only things that I've really hung on to (besides the sentimental) are my son's baby clothes and gear, in case we have another within the next couple of years. I tend to buy classic/conservative stuff, so neither the colors or styles would look outdated in a few years if there's a baby #2, and if baby #2 were a girl, at least for a couple of years, she can get away with the unisex-looking boy stuff. At least, that's my reasoning. But, if there's no baby #2 in sight in a couple of years...off they'll go to either a really close friend/family member if it's something I really loved on him or off to resale/consignment/charitable organization. Same for my maternity clothes that I haven't already parted with.