Please see the svn-buildpackage documentation for complete information. The important issues here are:
Since we keep upstream as a tarball we have to use svn-buildpackage in so-called merge mode. This means that, before compiling the package, the debianized source tree is constructed by untarring the orig tarball, and then merging the contents of the trunk subdirectory in it.
The structure of our SVN repository is not among the default structures of svn-buildpackage. Hence, we have to override the default location of some directories.
Place yourself in trunk/packages/my_packages/trunk, and do the following: svn propset mergeWithUpstream 1 debian. This registers the property "mergeWithUpstream" with the current directory, such that svn-buildpackage knows that it has to use merge mode as explained above.
Create a file debian/svn-deblayout with the following contents:
tagsUrl=svn+ssh://svn.debian.org/svn/pkg-ocaml-maint/tags/packages/my_package
Remember that "my_package" has to be replaced by the name of your actual package. svn-buildpackage will not include this file in the source package when actually building the package.
If you tried svn-buildpackage before writing your debian/svn-deblayout remember to delete .svn/deb-layout, since svn-buildpackages caches here the content of svn-deblayout (that would be empty in this case) and will ignore your debian/svn-deblayout.