There are basically four kinds of slogans:

Imperative Slogan - a command to do something:

– Advertise Here!  (Not very original but you get the message.)

Descriptive Slogan - describes the key feature or benefit of the organization or person: 

– Free ads here!

Superlative Slogan – exaggerates but in a way that makes a point and is something you believe to be true:

– Our ads have never been more free...

Provocative Slogan - makes the listener or viewer think or reflect on their situation:

– Free Ads? From What?

If you are trying to think of a slogan, a good brainstorming technique is to say essentially the same thing in each of these four ways and see which one has the best ring to it.

As an example:

Descriptive Slogans

- Neighborhood Services - Intelligent Changes for Our Community 

- John Jones - Bringing Positive Change to Our School

Imperative Slogans

Neighborhood Services -  Change Things

John Jones - Changes You Can Count On

Superlative Slogans

Neighboorhood Services - The Smartest Choice for Community Change

John Jones - The Most Intelligent Choice for Student Council

Provocative Slogans

Neighborhood Services - Do you really want anybody else making changes?

John Jones - What kind of change do you want?

Each of these slogans makes the point that Neighborhood Services (or John Jones in the case of an election) is a smart organization/person that will make some positive changes in the community. But each slogan is quite different and has a different ring to it.