Unfortunately, no good quick fix. Have to drop the headliner and recover. I just went through that on an e61 5 series. Not fun. But man does the suede headliner look good after all the work. Even added the m5 suede sun visors.

If you are just looking to physically hold the headliner up temporarily, then I've seen people use thumb tacks before. Not pretty, but it works.

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2005 X5 4.4i Build 04/05 Maintenance/Build Log
Nav, Pano, Sport (Purchased 06/14 w/ 109,000 miles) (Sold 8/15 w/121,000 miles)

2006 X5 4.8is Build 11/05 Maintenance/Build Log
Nav, DSP, Pano, Running Boards, OEM Tow Hitch, Cold Weather Pckg (Purchased 08/15 w/ 90,500 miles)

2010 X5 35d Build 02/10
Nav, HiFi, 6 DVD, Sports Pckg, Cold Weather Pckg, HUD, CAS, Running Boards, Leather Dash, PDC, Pano (Purchased 03/17 w/ 136,120 miles)