There are a couple of things that I think should be spelled out in big black letters - much like the health warnings on packets of fags - across the back of the packaging of Spanx and their ilk.
And I don't mean the obvious "These will probably help you to pull, but by God make sure you have them off by the time any friskiness rolls around unless you really hanker for a Bridget moment", like.
The first: do NOT wear a pair that encase your upper legs if you expect to be doing anything more active than swaying gently on a dancefloor or meandering between the bar and your seat. One day last week, I had to walk just shy of 10 minutes in some industrial strength undergarments and the feeling is only beginning to return to my thighs now. It felt a bit like wading through very thick soup while encased in too-tight medieval armour (well, it felt a bit like how I imagine that would feel.) Very bloody difficult and quite exhausting to try and move in something basically designed to restict!
And secondly: in much the same way that you'd lean forward and hoist your boobs into a push-up bra for maximum impact, you do need to do a bit of, er, lifting and seperating and "arranging" when donning your magic knickers to avoid a unibutt effect. Apart from being an aesthetic tragedy under clingy fabrics, this is very bloody uncomfortable - as I discovered on my return from a trip to the Ladies' where I'd forgotten to take my own advice.
I reckon that the memory of the incredibly numb bum that ensued from effectively being suspended in a sustained butt crunch for the evening will be reminder enough for me never to make that mistake again...