I have a pair of ER4-P earbuds and have wanted to try this for a long time. I finally gave it a shot yesterday, and it worked fairly well. I was able to clean old foam eartips so I could reuse them. Here's how I did it.
Purchase some hydrogen peroxide from the pharmacy. Pour a small amount in a bowl and toss in your used foam eartips. Squeeze each one a few times while submersed in the peroxide. If your's are like mine, you'll probably see clouds of disolving ear wax plume into the solution. Yuck! Squeeze 'em a few times and let them soak for 15 minutes or so. Then rinse with warm water using the same techinque.
Remove them from the water and Squeeze them dry. Then let them sit out in the air until dry. Be sure to place them so your cats or dogs can't get at them. You'll notice that they have a little more resilience than they did before and you can use them again with decent results.
They are not like new by any means but you should be able to get a few more weeks out of them.
I'm speculating here but I think that if you cleaned them every week like this, they might last quite a long time. My only concern is that the glue that holds the foam to the plastic tube may not stand up to the repeated soaking in peroxide.
Anyway, I hope this helps some of you extend the life of your foam tips a little and save a few bucks.
When I first got my ER4-S I used the foamies, but after only a week they were all distorted (shape). I then tried the tri-flanges and have not switched back since (about 6 months now). Unfortunately the longer lasting tri-flanges are not comfortable or do not give a good seal for alot of people.