I had this same problem with my old boy about three or more years ago and this is what I did.- - it is still as good as when I first did it.
I bought a couple of packs of what a 'Land Rover parts'seller called 'christmas/fir tree fasteners and then marked out a pattern on my sagging roof to make it resemble a 'button-backed chair'. I first pierced a SMALL hole in the roof liner with a stiletto at each marked point and then firmly pushed up a fastener into each hole. NB. I am NOT a DIYer but I do do a lot of craft work!! I was unsure just how much space there was between the roof and the liner so did imagine that I might have to cut a bit off the length of the leg of the faster but it didn't come to that as there was ample space up there. My fasteners are the same light grey as the liner but I can now only find black ones on ebay. I can't give you a link 'cos I am not computer literate with these things, but here are a couple of item numbers for you to look at, if you are interested, to get the general drift of what I mean:
311364999922, 381559655580,(good point on these), 321632781628, 311340750271 - no picture here though??