Once a headliner starts drooping there is nothing you can do to fix it. You can temporarily find solution though. I put thumb tacks in mine, they keep coming down every now and then though, you just need to push them put in. You could also staple it to the roof(not recommended).
The ultimate solution though is to buy a headliner kit for about $30 and replace it following the instructions.
http://www.stockinteriors.com/AutoHeadliner.asp?gclid=CP2_nJDHpYUCFRlWNAodVVc3tg
You can get things at a craft store like joanne fabrics that work fine and never fall out.they have like a clear button with a tiny like corkscrew on them so all you have to do is turn them in,they really work fine.hope this helps.
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