An easy way to do this is to use the OOB site provisioning provider that BLANKINTERNET uses (see example of this in stswebtemp.xml and accompaning site hierarchy manifest file).
An alternative is to use a custom provisioning provider in which you programmatically create sub sites (preferably in an XML file). Reuse the provisioning provider in the web template for the sub sites for creating hierarchies.
sharepoint provisioning provider
Another option is to use PowerShell. Define an XML structure that defines webs/subwebs (reuse the structure of the OOB provisioning provider if u like) and iterate it while creating New-SPWeb