How? Glad you asked.
Running Shoes : if you enjoy an occasional run, chances are that you have several pairs of shoes that get infrequent use at best. Rather than let them sit and gather dust, consider giving them to an organisation like Shoe 4 Africa or Soles 4 Souls; where they'll be cleaned up, sorted, and given to runners in the poorer parts of the world.
You get a clean cupboard, they get shoes. Everybody wins.
Clothing : although I'm talking about training clothing here, any clothing can be donated in the same way.
If - like me - you've inadvertently managed to accumulate a swag of workout gear over the years that no longer fits (t-shirts particularly), pass on the older models to places like Cancer Research UK, the Salvation Army or local charity shops.
Equipment : if you've been lifting for a while, there are probably a couple of items that don't get used all that often. Or perhaps you've just managed to land yourself a new rack, and there's a perfectly good one just sitting there.
Whatever the case, there are charities that specifically collect and redistribute this equipment. Of these, the largest is the Fitness4Charity group. Fantastic organisation.
NB : for smaller pieces of equipment and sporting goods, the Salvation Army is a great option.
Books : I've been surrounded by books for as long as I can remember, and an avid collector (and reader) for most of that time. To say I've got a few items in my fitness library is definitely understating things.
Given that, I'm always amazed when people sell their own precious strength-training tomes. Of course, if you're going to be getting rid of these gems in any case, why not donate them to Hands Across the Water, Books 4 Tanzania or your local library, school or gym.
Another option is to take part in one of the numerous fitness-charity events in your area. With everything from sponsored runs to indoor rowing contests, there's bound to be something that fits perfectly with your own training style.
Treat it like a meet or a major competition, and push yourself as hard as possible. Fantastic things.
There are thousands of events like this, held globally - here are just a few examples to show you what I mean :
Of course, if your financial situation allows, there's always the option of a monetary contribution. The organisations listed below are just a few of the ones I contribute to personally, as well as those which have been recommended to me. If you'd like to suggest an addition, contact me via email or twitter, or leave a comment below.
And, of course, those already mentioned.
As you'll see, they all employ strength-training to help make enormous changes in people's lives. If you've ever experienced these changes yourself, you'll understand just how powerful they can be.