The invention relates to improved means for preventing eversion and subsequent obstruction of thin-walled, floppy gastrointestinal liners implanted in the digestive tract of an animal. The implantable devices include an anchor adapted for attachment within a natural body lumen and a thin-walled, floppy sleeve open at both ends and defining a lumen therebetween. A substantial length of the sleeve has material characteristics that result in the sleeve being prone to eversion in the presence of retrograde pressures. Exemplary eversion-resistant features provide an increased stiffness and/or an increased friction coefficient between the anchor and the proximal end of the sleeve to resist eversion. In some embodiments, the eversion-resistant feature includes an anti-buckling element, such as a wire coupled along the substantial length of the sleeve.