To disable and enable ribbon button on change of Field Value using ribbon editor

Follow those steps:
1.      Click on the button
2.      Expand command
3.      Right click on command and select edit enable rule.
4.      Then click on the rule and press edit button.
5.      Write the field physical name in field section
6.      And also give invert (true or false) as your condition.
7.      In value section write the value on the basis of which button enable or disable.

Below Video is good :

http://www.youtube.com/watch?v=OE1CVePTeTc

Enable and Disable Section

Xrm.Page.ui.tabs.get('tab_name').sections.get('section_name').setVisible(false);

Enable and disable fields

Xrm.Page.ui.controls.get("fieldname").setDisabled(false); // to enable field
Xrm.Page.ui.controls.get("fieldname").setDisabled(true);// to diable field