Need to keep house cool in the summer - Help!!

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We live in North Carolina, if you have never been here, it's very hot and humid in the summer. We are currently renting a house that is about 100 years old. The problem is that last summer our power bill was like $400 a month, the a/c was turned all the way down however the house's inside average temperature was 80 We do not want to put in alot of money towards it but enough to save energy. Is there anything we can do? Like tape plastic around the window? Anything? My guess is that this house just has no insulation because our heat seeps out too.