The invention relates to improved means for preventing buckling and therefore eversion of thin-walled, flexible, 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 buckling and therefore eversion in the presence of retrograde pressures. Exemplary anti-buckling mechanisms provide an increased stiffness and/or an increased friction coefficient between the anchor and the proximal end of the sleeve to resist buckling and therefore eversion. In some embodiments, the anti-buckling mechanism is as a wire coupled along the substantial length of the sleeve.