I suggest you perform a system backup onto external media (an SD card, or through ActiveSync) before overclocking. There is a possiblity of damage, don't get me wrong, but it's so slim it's really not feasible. If you run your PocketPC overclocked constantly and forever then there will be damage, but there aren't enough days in a year to cause damage, so don't worry about it.
There are two programs, one free, another offers much more functionality but costs $10 at Handango.
The free program is called SpeedStepper (Or Turbo Tray):
http://elevatedstudios.com/pocket/
Just go there and click on "Download Speed Stepper." It's the free one that I have on my iPaq H3955. I never do anything that requires overclocking, though, so I never use it. I used it for benchmarking purposes. It's very good, though. If you select an unsupported speed your device will freeze. Simply soft reset to fix it.
The program that I would recommend, however, is XCPUScalar:
[url=http://www.handango.com/PlatformProductDetail.jsp?siteId=1&productId=53054&optionId=1_2_2&productType=2&catalog=30&txtSearch=XCPUScalar§ionId=0&platformId=2]http://www.handango.com/PlatformProductDetail.jsp?site...2[/url]
This piece of software is $10, however it is more than a simple overclocker. It allows you to set the CPU to scale to certain speeds based upon CPU usage levels. Thus you can set it to run at a mere 100MHz when you're simply looking up your contact information or reading an eBook and save TONS of battery life, but then as soon as you start up a game or play music or movies it will automatically scale to 300MHz, or whatever speed you wish to set it at. A very useful piece of software that gives you better battery life but performance when you need it. I'm hoping that in a later edition they will add the ability to set the CPU speed based on the program that is running, as well. However, if you have $10 handy, this program would be my recommendation. Be sure you try the demo version before you buy it, however, to make sure it works the way you want it to.