If you're not above case destruction (read: past end of warranty), cut more space for air to flow in your existing vents.
I had a friend who would remove the fan grilles on the back of his case. Since he was running under-voltaged fans (or low-power "quiet" fans) he wasn't afraid of touching them while they were moving. (I have extremely inquisitve cats, so I remove the case grilles and replace with a wire grille.)
You may also want to see about plugging up unnecessary vents as well. This sounds odd but if you can get a clear single path for air to flow through the case you can generally get very effective cooling.
Beyond that, you're starting to look at dryer ventilation hose and other such radical measures to direct air. My friends and I would use that to make the PSU fan pull double duty as the CPU fan (only in lower-heat machines like a Pentium II; I wouldn't do this on a heat monster like the Pentium 4 or Power PC G5.)
We usually optimized for noise not cooling, but you can usually improve both over manufacturer spec.
...or you can just submerge the entire thing in high-grade vegetable oil and run your cooling like that.