How can one define luxurious beds and seats in the eighth precept?
It's an interesting question; for lay people, I've never been able to well-define this one. I usually translate it as "don't get attached to the pleasure of lying down or sleeping." But, we should probably go with the Buddha's words for monastics in the Brahmajala Sutta:


  "Or he might say: 'Whereas some recluses and brahmins, while living on food offered by the faithful, enjoy the use of high and luxurious beds and seats, such as: spacious couches; thrones with animal figures carved on the supports; long-haired coverlets; multi-colored patchwork coverlets; white woollen coverlets; woollen coverlets embroidered with flowers; quilts stuffed with cotton; woollen coverlets embroidered with animal figures; woollen coverlets with hair on both sides or on one side; bedspreads embroidered with gems; silk coverlets; dance-hall carpets; elephant, horse or chariot rugs; rugs of antelope-skins; choice spreads made of kadali-deer hides; spreads with red awnings overhead; couches with red cushions for the head and feet — the recluse Gotama abstains from the use of such high and luxurious beds and seats.'
  
  http://www.accesstoinsight.org/tipitaka/dn/dn.01.0.bodh.html


I would venture to guess that modern spring beds of all kinds could arguably be included in this list.