Buying my add on products and beer donations are the best way to support the project. That revenue is what makes it possible for me to keep working on mojoPortal full time.
It is not required to keep the "Powered by mojoPortal" link though it is always appreciated and also helps promote the project. What I don't like to see is a site running mojoPortal that says "Powered by Something else". Something like "Site by YourCompany" is ok, but I don't like to see the software rebranded as someone else's product.
Technically I could not prevent anyone from putting "Powered by something else" on their web site but they would not find themselves in my good favor and I would not support them.
The license itself as with most open source licenses only covers re-distribution of the software, so if you are making it available for download or on distributable media you must do so in compliance with the license. But the license does not put any limitations on how you may use the software, so if you are not redistributing it, there is not much chance of running afoul of the license.
Also I would strongly encourage anyone working with the source code to keep any custom code in your own projects and avoid forking the mojoPortal code so that it is always possible for your customers to upgrade to a newer version of mojoPortal.
Other than that, any efforts to promote mojoPortal are always appreciated, things like blog posts or case studies of site implementations you work on using mojoPortal are helpful, even word of mouth telling others in your circle of influence about mojoPortal is helpful. There are still a lot of people out there who have not even heard of mojoPortal so anything that helps spread the word is good.