Well, there are a few. I found a great article here all about it, but for a quick overview, this is what I did:
For the dirty plugs:Soap and Water, Clean toothbrush (bleached or just ran in the dishwasher)
I had a pair that had sat out and gotten a little dusty. For these, I first washed them with soap and water, scrubbing the surface a bit with a clean toothbrush. For the toothbrush, just run it with a load of dishes, or if you used it for something not so pleasant, make sure to give a bleach soak.
For the waxy plugs:Hydrogen Peroxide Bath (or Witch Hazel less often)
For the waxy ridden ear plug, I put my pair in a small container of hydrogen peroxide for a few minutes. To keep them submerged, I just found a heavy round item smaller than my container and set it on top.
After the soak, I rinsed, rinsed, rinsed in water and let them fully air dry. This got the surface gunk off and they felt clean again. I've heard of people using witch hazel, as well, though I've read that witch hazel may faster dry up/make brittle some foams when used often over time. The hydrogen peroxide you get at the store is 3% concentration, usually, and hasn't been noted to have a reaction with foam ear plugs (PVC or polyvinyl chloride/memory foam) at less than 10% concentration.
For the "clean quick" plugs:Throw In Laundry
I've actually accidentally washed some ear plugs in the pocket of pants before, and they come out surprisingly clean! I'd say to put them in a pouch of some type- a small drawstring bag, a clean sock, pantyhose, a mesh bag - just so they don't clog up the washing machine or get lost. Between the detergent and rinse cycle, they come out clean and wax free (much like your chapstick will disappear inside a tube if washed.) If you are still worried about bacteria, just give them a quick hydrogen peroxide bath.
Although they won't last forever, and PVC in my ear all night probably isn't the best, until I find the 'perfect plugs', this is one way I don't have to spend money and still get my beauty sleep.
Places to get earplugs:Free:
Hospital or Hotel Stays - sometimes the foam earplugs are offered gratis as part of your stay - even if I don't use them during the stay, I tend to grab them to keep in my guitar case or to include in gifts for people.
Airlines - Big D made my day by grabbing his and his family's sets of complimentary 'sleep aids' from the airline they flew (Jet Blue, I believe.) Each contained a sleep mask and pack of ear plugs, and they are the most comfortable foam plugs I've worn!
Cheapest Otherwise:Harbor Freight - if you go through ear plugs a lot, like I did when I was in a metal band, buy a big box at Harbor Freight. With a 20% or 25% coupon, you can get about 50 sets for $3-4. They also have a fair reusable rubber style for $1.60 after your coupon.
The Dollar Store - for $1 you can get 1-3 sets of the foam, and sometimes a not-half-bad reusable rubber style.
Clearance aisle in the fall (from swimming area) - Although meant for keeping out water, you can get reusable ear plugs for $.25-.50 after the swimming season is over. I've seen them at grocery stores and Walgreen's on clearance for less, sometimes!