So here is Little Miss's dress before blocking:
The results are great!!
I used my 'Penguin' steamer as they are nicknamed. It's not much use as a cleaning steamer but it's great for defrosting the freezer and as I now know, for blocking/killing acrylic.
And after being 'killed' by a penguin:
The drape is much better, the fibres are much more relaxed - they don't spring up in odd places - apparently it's good to be killed by a penguin if you're an acrylic dress ;-).
Killing an acrylic project with a penguin steamer is easy. Just look at Beadknitter's blog for a tutorial on killing acrylic. I didn't pin the dress in place before killing it. I simply held the steamer about 4 inches fromt he dress, steamed it section by section and smoothed it with my hand as soon as the section was cool enough that I wouldn't burn myself. When you steam in this way, you'll know when you've steamed enough as you can see the yarn change appearance. I wouldn't go beyond this point or the acrylic (which is a plastic) might melt too much. I'm guessing that killing acrylic is basically melting the plastic enough to make it relax but not so much that the garment becomes a puddle of melted plastic.
Oh and here's a picture of Little Miss modelling - somewhat reluctantly - her dress. I decided to add a crocheted trim after killing the dress - I'm sure more organized, sensible people would have done it the other way round but the trim was an after-thought.