Can you ask a nameserver to refresh its cache for a given name?

[andy baxter]

hello,

Is there a command you can use to ask your nameserver to refresh its 
cache for a given name? The reason is that a web (phpbb) forum I go to 
has just changed to a different server, and they are saying not to post 
until the name is pointing to the new server. Is there a command I can 
run here to hurry this up, or do I just have to wait? I've looked for 
options to do this in host and dig and not found any.

andy.