My advice, sweet talk and honesty with the attendant. That almos always works for me, I am just honest and tell them i'd really appreciate any upgrades possible and if they can't 'no worries', the low pressure technique and being nice even if they can't seems to work with 'most' of the National ppl i've dealt with. Last week I had an XC60 Gxar from a midsize rental and the week before that a loaded 14 Acadia on a midsize rental from the EE in DFW.