Potential Solutions...

If the speed you're trying to run at won't work for some reason, then some of these quick-fix solutions may help...

1. Add additional cooling if your CPU is getting too hot. A fan blowing on the fan that's on the CPU can help. You could also try a bigger/better heatsink and fan combo or a Peliter effect cooler which basically works as a refrigerant for your CPU. Using thermal grease to seal the connection and enhance heat transfer between your CPU and heatsink can also be very helpful.
2. Though it can be dangerous if you go too high, you may want to try increasing the voltage.  Not all motherboards support this option though.  Do so in small steps if you can and avoid going more than four or five tenths of a volt higher.  Make sure your CPU isn't getting too hot as you try this.
3. Sometimes the CPU's cache won't tolerate the higher speeds.  Although I wouldn't suggest this to increase performance because it would have the opposite effect, you can disable your CPU's cache in the BIOS just to try to find out if that is what is preventing you from reaching a higher speed. Then change it back later.
4. If you're running a Pentium II and it won't let you run at the 100 MHz bus speed or higher, try to cover up the B21 pin on the CPU. Tom's Hardware Guide has a good description of this. The same applies to the Celeron CPUs.
5. If you have an older hard drive ('97 or earlier), consider lowering the PIO mode in the BIOS as the hard drive may not like the faster bus speed.
6. Adjust memory timings in your BIOS.  Take them as low as you can without losing stability in an UNoverclocked situation, then try them in an oveclocked situation.  You may have to have slower speeds for your memory to be able to overclock (i.e. run your RAM at CAS 3 when overclocking and CAS 2 when not). CAS 3 would be much slower, but it may allow you to run your CPU faster (and tell you to get faster RAM that will run the higher bus speeds at CAS 2.
7. If you run one of the first Athlon CPUs, look for a "Golden Fingers" card to allow you to overclock the CPU. 
8. Here's a long but important note on PCI and AGP cards. If, for example, your system runs at 133 MHz bus speed by default, then increasing your bus speed to 166 overclocks your RAM, PCI cards, and AGP card.  At 133, your PCI cards run at 1/4 the bus speed (33 MHz, which is default).  At 166, the divisor may remain at 1/4 in which case your PCI cards are overclocked to 41.5 MHz. This may be okay for most PCI devices, but some may have problems.  See if your motherboard has a 1/5 divisor for PCI (PCI would then run at the default 33 MHz).
   For AGP (usually more important because AGP more often has problems with higher speeds) the problem is similar. AGP is supposed to run at 66 MHz. At 133 bus, AGP is configured by 1/2 the bus speed (133 * 1/2 = 66).  When running at 166 MHz bus speed, your AGP card may overclocked to 83 MHz, which may be too much.  If a ratio of 2/5 is supported by your motherboard, then the AGP speed would be at default (166 * 2/5 = 66 MHz). 

Check back occassionally as I'll add other suggestions as I think of them. If you have a suggestion, email it to me...