Which interface do you use? I prefer psvn, which says the following:
Q3: How do I enter a username and password?
A3: In the svn-status buffer press "s", switch to the
svn-process buffer and press enter. You will be prompted for
username and password.
As far as I know, psvn does not provide a way to save your password. Anyway, it would not be smart to store your password e.g. in your dotemacs. It's a good idea to use SVN over SSH since the command-line Subversion client (the one that psvn uses, too) can remember HTTPS passwords. In that case, you may want to generate an SSH key pair that keeps you typing your password every time. There is an entry about it in the Subversion FAQ how you can do that.