The headlining was sagging in my P38, so back in August I thought I'd do something about it as a temporary fix. Next year I'm planning to get a new headlining.
Anyway, to do what I did below you need a staple gun, some small round velcro patches (sold in Halfords) and some grey plastic trim clips (ebay).
1) Cut off the plastic lug from the back of each of the clips so they are more of a round disc.
2) Stick one half of the velcro strip on the back
3) Staple up the other half of the velcro patch using the staple gun into the headlining where required.
As the clips are held on with velcro, you can remove them and re-staple if needed
I've attached a pic. Hope it helps.