Over the course of many years I have managed to create blogs on many different sites: xanga, lj, and myspace. How can one go about taking the data from these services and moving it to a new service, ie blogSpot (which may not even be that new)
Assuming there is no standard export file from your old blog, and an import routine in the new blog system that can work with it, the only idea I can think of is getting the old blogs RSS feed (assuming you can make it give an RSS of all the posts, something that is not always possible). You will loose all your comments BTW.
I know that WordPress (the self hosted version, that is wordpress.org) has a system (the plugin feedwordpress) that will allow you to import and RSS feed into a new wordpress blog.
Then from there on you could export the wordpress file into most common system that exist today.
A lot of work for you...
Good luck, Tal
There are no general rules for doing this. It is all in the hands of the blog platform developers, blog platform supporters or in yours.
For example, if your target blog is Wordpress, you can take a look to this page: Wordpress » Importing Content.
For this reason is really important choose carefully the platform where you plan to initiate a new blog.