So I fixed SF40 SuperFOIL to the underside of my rafters to keep warm, cut bills and save the planet: www.superfoil.co.uk/SF40-titlepage.html
I chose SF40 because:
a) It nails to the roof, so you can still use the loft for storage
b) It has a radiant heat barrier
c) It is the equivalent of fitting 450mm (18 inches) depth of fibreglass*
d) It should keep us cool in summer too
I expect there are other products you could use, but this one seemed right for us.
There are a lot of steps to this instructable, but it's not as complex as all that. I waste a lot of time talking about nailguns and such. You can skip to step 16 if you want.
NB: Apparently what we Brits call a loft is what our Yankee cousins call an attic. For them this 'ible is about attics.
* Clarification: fibreglass does not block radiant heat; the quilt does. In order to get the same savings, you would have to install 18" of fibreglass. But this does NOT mean that the R values are the same. See the manufacturer's brochure illustrated in the second pic of step 3.
Step 1: And here it is
You can use insulating boards; but they don't give the same insulation value, and you can't cut them with a pair of scissors.