I am willing to sponsor packages, but generally not willing to take over the maintenence. I've got too many other things going on, so I would do a bad job if I tried to maintain more packages.
When sponsoring, I prefer being approached by the package maintainer on IRC (pere on irc.debian.org and irc.oftc.net) with the URL to where the package can be downloaded. I prefer an URL to a .dsc file on mentors.debian.net to pass to dget. This is a low-overhead solution to me, and I do not accumulate a large backlog of request for sponsoring (as would happen if I tried to gather sponsor request using email. :).
The packages should be lintian clean and tested in pbuilder or similar, to make sure the dependencies are correct and obvious problems are found. I primarily sponsor packages going into debian/main, so if the package isn't acceptable for main, there is normally no need to approach me.
I find it best to hear directly from the person planning on maintaining the package, to be able to ask how much time the prospective maintainer plan to spend on the package, as well as work out issues with the package directly.
When I sponsor packages, I add a line 'Upload sponsored by Petter Reinholdtsen' to the debian changelog to document the fact to the next sponsor of the package.