It heavily depends on what your product is, but you've at least these possible models:
1) Fully open source with lack or light documentation. This makes your product essentially free but users pay for support and/or the docs. I can't remember any specific example of a project selling the docs but I'm sure someone will.
2) Dual License model. A very popular example is ExtJS which is GPL (v3 iirc), however, if you wish to keep some code secret (including server parts) you might need a commercial license. And of course there are support plans available, as well as SVN/GIT access to the latest (devel) version.
3) Dual License with a Enterprise version. Essentially what MySql does where they offered an open source version but if you wanted fine tunned performance, support for enterprise hardware and support then you need the Enterprise version.
4) Dual License with long term support. Some projects like Liferay or Red Hat Enterprise use free versions as beta versions - after a while they release a long term supported version for enterprises and backport the important security and bug fixes. Maybe you already know but some companies are very slow to adopt new tech and ever slower to keep up, if they can keep a 4 year old version of the software that does the job well and still get support and bug fixes, you're best pals.
5) Early access model. Another possibility is to offer early access to new versions. For instance, the Xming project (a X11 server for Windows) offers donators access to new versions much earlier. You can even create a "pool" mode where you release the new version once X dollars are donated.
Depending on your target audience and the possibility of some of the adjustments required by those suggestions you might find a suitable model or cook some solution with ideas from several.
From someone in a similar spot, I wish you luck!