First step would be to open up the machine and give it a good spring clean, making sure you clean out the dust and fluff from all the fans and fan housings. (especially PSU) Give the entire machine a good blow out with a can of compressed air to get rid of dust, fluff and spider webs. Check the location of your disk drive(s) and ensure you have enough space around it/them to enable adequate air flow. Replacing flat drive cables with round ones can also help considerably in ensuring decent air flow from case fans.
Best situation regarding case fans is to have two that are opposite each other, one extracting the hot air, one pulling cooler air in