Changing the bus Speed
To understand how you can overclock a Pentium, Pentium Pro, 6x86, or K5 CPU, it helps to realize that the internal clock in these CPUs runs at a different speed than the external clock or bus speed. The external clock is the speed at which the cache and the main memory run and when divided by two yields the speed of the PCI bus. There are only three different official bus speeds used by the Intel Pentium, Pentium Pro, and the AMD K5 CPUs - 50, 60 and 66 MHz. The 6x86 uses five bus speeds: 50 MHz, 55 MHz, 60 MHz, 66 MHz, and 75 MHz. There are also new boards available which support the unofficial bus speed of 83 MHz.
To change the bus speed, look in your motherboard manual for something like 'CPU External (BUS) Frequency Selection' - these are the jumpers you will have to change. If you are lucky and happen to have a motherboard with the new SoftMenuTM technology, you can change these settings in the BIOS setup menu right from the comfort of your chair. You don't even have to open the case.
Always go slowly and increase the bus speed one step at a time (e.g. go from 60 MHz to 66, not 60 MHz to 75). This is the most successful way to overclock. Using this method, almost every P150 CPU runs at 166 MHz and most all 6x86 P150+ CPUs run at a P166+ level or 133 MHz.
Changing the Multiplier
The internal clock is controlled by an internal clock multiplier in each CPU which is programmed via CPU pins. Intel Pentium CPUs support the following multipliers: x1.5, x2, x2.5 and x3. Intel Pentium Pro CPUs support x2.5, x3, x3.5, x4. 6x86 CPUs so far only support x2 and x3, but the upcoming M2 will support x2, x2.5, x3, x3.5. The K5 is kind of a difficult fellow here, because it doesn't seem to be affected by the external settings of its multiplier. So far it only uses the x1.5 multiplier for each of the PR75, PR90, PR100, PR120, PR133 CPUs. The new PR150 and PR166 K5 CPUs will use the x2 multiplier, but it seems fairly likely that you won't be able to change it - although I have not been able to verify this.
To change this setting, find something like 'CPU to BUS Frequency Ratio Selection' in your motherboard manual. There are usually two jumpers used to change these settings. Again, you can do all of this in the BIOS setup menu if you have a SoftMenuTM motherboard such as the new Abit motherboards.0
Changing the CPU Supply Voltage
You might not like it, but this is something which is often required to make the CPU run more reliably.
First of all, I'd like to stress that the Intel Pentium and Pentium Pro CPUs can run at a supply voltage of up to 4.6 V. This, of course, requires serious cooling because the chip is producing a lot more heat than usual. I have tried this with my own P166 and the chip is still doing fine. It didn't really help my problem either, however, since it was the memory that would not run reliably at 208 MHz, not the CPU.
Often the change from STD to VRE voltage is the whole trick to successful overclocking. This is due simply to a bigger voltage difference between the digital HIGH and LOW conditions, which results in 'cleaner' signals for the CPU and other motherboard devices. If you can't run your CPU reliably at one particular clock speed, it's always worth considering changing to a higher supply voltage. The silicon of STD and VRE CPUs is identical, so you won't damage your STD voltage CPU with VRE voltage, the CPU will only run a little hotter. The Abit IT5 motherboards offer a voltage which is even higher than VRE, of 3.6 V in their SoftMenuTM BIOS CPU setup. I'm running my CPU at this voltage and it runs completely stable at 205 MHz @ 2.5 x 68, even as I type this document.