If you can, spin down the hard drives. This will save a bit of warmth. It's possible to do this from the control strip on some machines (it made a huge difference on my Clamshell), but on others you'll have to find other alternatives.
Using programs that don't access the drive much is also a positive. I have a 180c and usually use MacWrite II on it for word processing. I find it hardly ever uses the drive during normal typing and makes the computer comfortable enough to perch on my knees (I never set computers on my lap). It's a little harder to control these sort of things when you're using virtual memory, so turn it off on any pre-OS X machine.
I found the hottest computer I ever used was an iBook G4 (as I've said before on here, probably the worst Mac I owned and definitely one of Apple's worst in history for reliability and performance). The 100 series isn't too bad; the 180c is a little hotter than some of the others (I've used the 150 and 170 extensively and found both a bit cooler). The clamshell runs cool, my Santa Rosa MacBook is OK as long as it's not taxed, and my old DOS-based Toshiba T1200 was perhaps the best-cooled laptop I have ever used (and that design came out in 1987).
5300s are garbage in every respect except performance. Apple made a lot of junk computers from 1994-1996, and the 5300 was definitely one of the worst. The price was raised by the fast processor, but the quality of the machine itself was poor and the batteries, of course, were infamously recalled not long after it came out. The 190 suffers from some of the same quality flaws (especially with the case plastics) but at least it ran cooler and wasn't a terrible computer for the price. Overall, not a bad machine, definitely better than its PPC cousin.
Here's a tip to cut back on warmth and skip an unreliable component in 1xx series PowerBooks--take out the hard drive, install a decent amount of RAM (4MB minimum), strip down System 7.0.1 or 7.1 with the "mini install", use a RAM disk, load the system to the RAM disk, and run your programs from floppies. You'll be limited to older/smaller programs, but you won't have a hard drive creating extra warmth plus you won't have to worry about your SCSI hard drive conking out at an inconvenient time. I'm currently fixing a 140 to run this way (it has a slight display problem that needs fixed and has no hard drive, but the floppy works fine).