Woah, didn't know there was a thread just for this
I too have found it literally impossible to achieve a good seal wearing the IE8 the "normal" way. The right earpiece, in particular, after a while just slides out enough to ruin the seal - and the bass. I can get a good seal with foamies, but I never was a fan of foamies sound-wise. Also, fitting the Comply tips on the nozzle isn't easy.
Wearing the IE8 with the cable down is, inexplicably, just better for me - and I see I'm not alone, ha ha
I say inexplicably, because they go into my ears exactly the same way as wearing them over-the-ear. It's probably the angle of insertion that makes the difference.
But the funniest thing is, it's even better wearing them cable down but with the earpieces inverted - L in my right ear, and vice versa. It's aesthetically terrible, but the fit is solid!
If only Sennheiser made the earpiece like Westone's or Shure's. The UM2 is still the best-fitting IEM I've ever tried.