You can fill a washing up bowl (or bathroom sink?) with hot water and a bit of bleach and squeeze this solution in and out of them a few times. Then rinse off in hot soapy water (washing up liquid etc) and make sure you have got all bleach out by squirting plain water in and out a few times.
But they do go manky after a while and as they're quite cheap, probably easier to just replace. I think the bleach thing works better as a preventative than a cure.