Firstly, with every package revision, you have to edit the changelog. This is a requirement if you make changes to the package; you can add such changelogs with dch, as Seth suggests.
However, if you are simply trying to produce a package that has no additional changes, so you can just install the package, then you don't need to edit the changelog, you simply need to resolve the signing key issue.
I don't believe either of the answers here is 100% complete. Therefore, I will steal slightly from both, but add my own suggestion and solution here, as I do this with the nginx package merges quite often.
To quote Seth, debuild will determine the key based on what the last changelog editor was. This is automatic, and you will need to update the changelog to use your credentials in them at the end of the latest changelog entry.
As was stated by Florian, though, you can also use the -kKEYIDNUM option to debuild tell it which key to sign with, and enforce the use of that key.
And now, my solution to both issues, to make things automatically sign with the key I want to sign with...
For the longest time I ran into this issue whenever my old hard drives died on my previous system. I did not want to edit the changelog each time, really, nor did I really want to manually pass the -k option each time to debuild.
Finally, MOTUs helped me solve the problem by explicitly specifying what key to sign with, by introducing me to .devscripts, which debuild and others call upon environment variables with things defined in them; this permitted me to add options that dpkg-buildpackage, which debuild calls, will always append.
So, to make the -k option work automatically for every single debuild you run, you can add this to you ~/.devscripts file, and automatically add the -k option, like so:
DEBUILD_DPKG_BUILDPACKAGE_OPTS="-kABCD1234"
This will make it persistently added to the debuild options; this is also a way to enforce that your key will always be used for signing.
This helps me for both Ubuntu uploads, but also for PPA uploads.