Lots that you can do. How much have got to spend?
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Get some top quality fans, a quiet CPU cooler, some silicone fan mounts for the case fan and some silicone case feet. Your graphics card and motherboard northbridge fans - if present - can usually be replaced by some quality heatsinks.
Your case fan should be of the large, silent and dust-proof variety.
If your budget allows, you really should line your case with accoustic foam and change your PSU for something like a Q-Technology Gold Series.
Things that cost little or no outlay include checking every screw and mounting in your case, positioning it on a very solid and stable surface, as well as keeping the interior and fans dust-free. Spotless, preferably.
Study the direction of airflow; is there more than one channel of air? Do they cross or otherwise interfere with each other? Are your fans pushing in different directions, such as 90º to each other?