Keeping citizens happy
To keep the citizens happy with you as mayor, lower taxes to zero. Just before the end of the fiscal year (December 31), pause game play and raise taxes to the maximum. Then lower them again in January. Taxes are not collected until the end of the year, so they do not have to be raised to the maximum until that time.
Save money while deleting highways
When you are deleting highways, just destroy one half of them. You will save five Simoleons for each square.