After getting the tarball from postfix.org and untaring you first need to run make (there is no ./configure available in this one). You might hit an error that sounds like:
No <db.h> include file found.
Install the appropriate db*-devel package first.
In this case (at least on CentOS 5) install db4 and db4-devel using yum
yum install db4
yum install db4-devel
After that run make and add 2 system users: postfix and postdrop.
make
useradd -s /bin/false postfix
useradd -s /bin/false postdrop
You will have to provide a number of paths (if you want them different from defaults. These are the main defaults:
/etc/postfix # config dir
/usr/sbin/ #administrative binaries
/usr/libexec/postfix #postfix daemon
/var/lib/postfix #cache and other random data
/usr/bin/mailq #Sendmail compatible mail queue listing command
/usr/bin/newaliases #Postfix newaliases command
/var/spool/postfix #Postfix queue directory
/usr/sbin/sendmail #Sendmail compatible mail posting interface