i second taking her outside and getting fresh air... it does seem to tire them out....

it works a treat with my 13month dd, we take her outside to play on the slide, sandpit, cubby, grass, whatever, all the fresh air and excitement of exploring somewhere so different from the boring loungeroom seems to really exhaust her... obviously at 8 months you can't do the same thing, but maybe a rug outside on the grass with some crawling and exploring the texture of the grass, point out the trees and different objects and she will learn the names of things and develop language and start to point at the objects when you ask her to... also, have you tried swimming - another trick to exhaust them... maybe a splash in the local heated pool and then home for an afternoon nap... who knows worth a shot!!

good luck, it gets easier!!