I've received a few requests to help with failing builds in the past, mainly for packages I maintain (either officially or unofficially). And most of those requests revolve around custom Wine builds. Because this is recurring, I'd like to take the opportunity to write down some general answers which solve most of the problems, so I can point people to this entry.

• Use pbuilder (cowbuilder is a good companion to pbuilder).
• If you've modified the packaging, make sure you didn't introduce bugs, which lead to your failure (this implies careful reading of your build logs).
• If you didn't update the packaging and just used a newer upstream version to build your packages, you should check what was changed upstream (maybe you need some additional Build-Depends or something similar) and whether the unmodified Debian package (including the .orig.tar is buildable in current Sid.
• If you added patches (maybe taken from some upstream bug tracker), make sure it's not those patches, which cause the breakage.
• When you do a backports build make sure all build dependencies are satisfied.

Apart from that, you'll most likely find a lot of answers in the Developer's Reference and the New Maintainers' Guide.

Nothing of this is news to Debian Maintainers, but I hope it helps people doing a build for themselves, when they're less experienced.