32M /boot
2*RAM for sawp
~30M /boot
250-500M /
250 /tmp
250-500 /var
enough on /home for email, etc.
The rest on /usr
Then, I chown /usr/local to my main user and use that for installing dev software (/usr/local/dev), apps (/usr/local/apps), downloading things (/usr/local/dnl), and source code (/usr/local/src).
Of course, Karsten will now tell you that my laptop has everything in one partition in complete contradiction to the above advice. :-)