# yum --enablerepo=rpmforge install some-tool
Of course it was inconvenient, I had problems with updates and so on.
Then, a single look on the yum documentation showed me the yum-priorities package.
Now I have all the repositories enabled and prioritized. For example, the [base] repository has priority=1 and [rpmforge] has priority=20.
This way, yum update will not replace httpd with the latest version from rpmforge, and yum install htop will succeed. Of course, you can make it in another way. For example, you can add a PostgreSQL repository and make it higher priority than base/updates - this way you'll always keep your postgresql packages up-to-date using just yum update without a need to upgrade it manually.