First you will need to buy some fans. Newegg has them for around $3.00
Next you'll need a molex adapter to power your fans. Newegg for around $15.
now you'll need a timer. I got a GE 7 day digital at home depot for around $15. It has 20 on/off cycles a day so you can run your fans every 70 minutes or so depending on your preference
Now you'll need something to mount to the back of the fans so that air can flow around. I used cabinet door stops attached with gel superglue
Depending on your cooler find a safe place to drill a access hole for the fan wires. Mount your fans where you like, cut off the molex connector so you can run the fan wires through the hole you drilled. Splice the wires together on the backside of your cooler into the molex connector
Plug it all in, set up your timer, and you should be good to go.