A staking system like this would be effective

http://www.ehow.com/how_4829327_stake-perennials.html

Place the system in place in early spring and the plant will grow through it and maintain shape.

Are they in sun or shade? I've found that if they're in shade they tend to grow taller, and cannot support their own weight. As A/man says, a neat staking system, something unobtrusive, would sort it out, or some short 'y' shaped hazel twigs. Phil

When they collapse I'm ruthless and remove the old growth. I'm in two minds about staking - I hate the poor plants to be tied to stakes like missionaries or dissenters about to be burnt alive. :-( On the other hand I do accept that some tall things have to be tied up - I just don't like the look of it - so I don't grow them any more.

Also with the staking system I linked to above you could push it quite deep into the groujnd in early spring so it is unobtrusive and raise it up a little as the plant grows.. that way it would remain almost invisible

I find, as stated above, that if I stake things early in the season, with short canes and a network of string stretched across the growth, the plants don't look "staked" as they grow up through. I agree that if you wait until they have flopped over before you stake them, they look awful!
I use those short green canes that you can buy in the Poundshop for short, top heavy plants like sedums.

I agree with philljefs, i find in full sun they do better, i have quite a few on my allotments, all in full sun, however occaisionally i have had this problem myself, i find this is possibly due to the crown being to large, but once divided they seem to be ok the following year.

Thank you! I have one in sun and one in shade and they behave about the same. I'll look in the pound shops for some stakes. I tried cutting one of them down to see if it would grow back more compact but that didn't really work, it grew back very straggly.

Very easy to propagate, division, or in the spring take some of the shoots at the base off when they are aprox an inch long and pot up, or even some of the leaves in the summer take quite easy.