I haven't tried this myself, but I just found this trick in an Amazon review of a DE razor:
"How you may ask, do I sharpen a double edge blade? Easy... Take a drinking glass (key word is GLASS not plastic), fill it half full of warm or hot water, take the blade and carefully slide it down the inside edge of the glass with your index finger until it is submerged, then rub it side to side about a dozen times, take it out, flip it over and do the same to the other side. You're good for another 10 to 12 shaves!"
Just to give the proper credit and not be accused of plagiarism, the source was:
Amazon.com: Ray C's review of Parker 66R Butterfly Open Double Edge Safe...
"How you may ask, do I sharpen a double edge blade? Easy... Take a drinking glass (key word is GLASS not plastic), fill it half full of warm or hot water, take the blade and carefully slide it down the inside edge of the glass with your index finger until it is submerged, then rub it side to side about a dozen times, take it out, flip it over and do the same to the other side. You're good for another 10 to 12 shaves!"
Just to give the proper credit and not be accused of plagiarism, the source was:
Amazon.com: Ray C's review of Parker 66R Butterfly Open Double Edge Safe...