It's all pretty much been covered here, but the one thing you can't teach is to lose the fear, the fear that'll force you to brake, to tense up, to look at the roots.
I'm guessing you used to ride them without worry before you stacked..remember those times, you know you can ride them, try and get your mate to session a section telling you what he's doing at what point on the trail, sometimes it's natural and he maybe unaware what he's doing right, but look at him and not the bike and try to see where and how he's unweighting the bike. look at at from various angles.
Practice unweighting the bike over smaller less slippery obstacles then progress to small roots, then rootier sections, in some cases you can jump clear of the main section of roots by using the first root as a kicker.
Start small and keep trying.