For Windows XP, Vista and Windows 7:

1. Open the Start menu.
2. Go to Run. (If you do not see the Run command search for "run" in the Search bar)
3. In the Run text box type: ipconfig /flushdns
4. Press Enter and your cache will be flushed. 

For MacOS:

1. Go to Applications.
2. Go to Utilities.
3. Open the Terminal application.
4. Type: dscacheutil -flushcache
5. Press Enter and your cache will be flushed.

For linux:

1. Google a solution :)