The Raspberry Pi is a nice piece of hardware, but sometimes it is a little too slower. In my case, I feel it every time I navigate with my TV remote the XBMC UI of my OpenElec installation. So a couple weeks ago, I tried to overclock the raspberry pi hoping to have a little boost in performances while not breaking it.
After two weeks of intense usage, I did not found any issues and I am quite happy with the increased performances. Before starting, the usual disclaimer: I am not responsible for any damage you may do to your board following this tutorial. Do it at your own risk.

The procedure is very simple, you just need to access your SD card from a pc and modify a text file.

First, open the SD card with your OpenELEC image from your pc. You should see the same as in the picture:

Now open the file config.txt with a text editor, go to line 54, uncomment the four lines (arm_freq, core_freq, sdram_freq and over_voltage) and change their values with the ones taken from the comments you find just before them.

# Overclock mode settings.
#
# default recommended values are: arm_freq | core_freq | sdram_freq | over_voltage
# no overclocking               :    700   |    250    |    400     |      0
# mode 'Modest'                 :    800   |    300    |    400     |      0
# mode 'Medium'                 :    900   |    333    |    450     |      2
# mode 'High'                   :    950   |    450    |    450     |      6
# mode 'Turbo'                  :   1000   |    500    |    500     |      6

 arm_freq=900
 core_freq=333
 sdram_freq=450
 over_voltage=2

This is the setting (medium mode) I am using and, until now, I did not have any problem.
Feel free to experiment, but remember that the wrong settings can eventually compromise your board.