quick example:
Dear School you want more money out of,
Thank you for the acceptance and the generous aid offer. Although your school is my number one choice, I simply can't afford it with the current aid package offered. Are there any additional scholarships or aid that I can be considered for? I have recently received offers of $$$ from school X, $$ from school Y and $$ from school Z. I would love to spend the next three years at your school but if the current aid package cannot be increased, I will be unable to attend your school. Thanks.
Something similar to that. Polish it up and make it sound nicer.
I wonder about doing this. The thing that I am afraid of is that if they don't want me bad enough to give me any (more) money, the school will get PO'd. I want to stay away from If->Then statements, because I don't like the contrapositive (or is it inverse? the LSAT was a long time ago.)