mod_macro - it's ace. I found it last month, we're using it on live and it's made our lives so much easier already.

(The author is absolutely determined it will not become Turing-complete - it does only very stupid substitution. He says if you want anything fancier, use Perl and abandon all hope.)

We have a pile of stuff like this in our apache/conf/ directories:

vhost-default.conf (relevant section, at end):

...
<Macro FOO_LOCAL_CUSTOM_LOG $APPLICATION $DOMAIN>
CustomLog "|/usr/local/apache/bin/rotatelogs /data/apache/$APPLICATION/logs/$DOMAIN_server0000123.server.not_access_log.%Y-%m-%d 86400" withcookies
</Macro>

<Macro FOO_SERVER_ALIAS_FARNARKLE>
ServerAlias farnarkle-04.host.example.com
</Macro>
Include conf/vhost-farnarkle.conf
...

httpd.conf (load the module, define the macro, use the macro)

 ...
LoadModule macro_module       modules/mod_macro.so
 ...
Include conf/vhost-default.conf
 ...
Include conf/vhost-farnarkle.conf
 ...

vhost-farnarkle.conf:

 ...
Use FOO_SERVER_ALIAS_FARNARKLE
 ...
Use FOO_LOCAL_CUSTOM_LOG farnarkle www.foofarnarkle.cam
 ...