The company I work at has highs and lows regarding work hours. We are very seasonal.
There are months when there is a lot of over time available and there are months
I may not even work 20 hours. My company hires heavy when we are busy to keep overtime
down. Then when we are slow the excess help is let go but still leaving hours for the remaing
employees less than 40. I would be willing to work 20 extra hours a week and have my company
save those hours for weeks when we know we will be slow. My company would be willing to
do this as well however we are unsure of the laws. Are they required to pay time and
a half for the hours I work over 40 if it is agreed that the only reason I am choosing these
hours is to provide income for myself at a time the work will not be there. At the same time
this will provide me with more security as they will not need to hire so heavy during the busy
season.

I do not see an actual question in there. If you are asking if this practice is legal, the answer is that it is not unless your employer is governmental in nature, and even then, it would work differently then you describe.

Past that, we have had posters who had such practices (even though the practices are not legal). The poster's problem was that the unpaid overtime was supposedly "banked" but when things slowed down the employers just laid off the employees anyhow and kept the unpaid overtime.

http://www.dol.gov/esa/whd/regs/compliance/whdfs23.pdf

Overtime Pay May Not Be Waived: The overtime requirement may not be waived by agreement between the employer and employees. An agreement that only 8 hours a day or only 40 hours a week will be counted as working time also fails the test of FLSA compliance. An announcement by the employer that no overtime work will be permitted, or that overtime work will not be paid for unless authorized in advance, also will not impair the employee's right to compensation for compensable overtime hours that are worked.