I'm building only a limited number of packages, which have the same dependency set on every build. Unfortunately, pbuilder tries to install the dependencies on every rebuild instead of storing a prepared image and reusing it every time, for example.
Is there a way to enable such functionality? Can I somehow skip the long step of installing build dependencies if they didn't change between subsequent builds?
Yes. You can chroot into your unpacked pbuilder environment using pbuilder --login and install whatever packages you like. Normally, all the changes you make there are thrown away again once you log out, but there also happens to be a --save-after-login switch which will cause all changes made to be included in the pbuilder chroot you logged into.
When doing that, I highly recommend to first copy the environment you'll be editing and only edit the copy. That way you'll still have a "clean" environment around that you can use to check that your packages correctly declared all their build-dependencies before uploading them to some archive. You can use pbuilder's --basetgz option (or --basepath when using cowbuilder) to switch between different chroots.
