Assuming you mean automatically launch the daemon at boot time, you can put a script in your /etc/rc2.d directory called Snn<scriptname> which would contain the following:
su - <user> -c "<daemon_start_script>"
nn would depend on how soon you wanted the daemon to start after entering the startup procedure (1 for immediately, 99 for the last event etc). It might also be an idea to include the line:
. /<users_home_dir>/.profile
early on in your script so that any environment variables are set correctly. I've just realised I've answered this from a Solaris point-of-view - hopefully the same will apply to Linux variants. Others may be able to confirm this. Cheers.