Setup de Grunt DEMO, para ver toda la explicación, está en http://www.zerothedragon.com/686/grunt-es-lo-que-necesitas/ :D

Gruntfile.coffee

module.exports = (grunt)->
    grunt.loadNpmTasks 'grunt-contrib-watch';
    grunt.loadNpmTasks 'grunt-yui-compressor';
    grunt.loadNpmTasks 'grunt-contrib-less';
    grunt.loadNpmTasks 'grunt-contrib-clean';

    grunt.registerTask('default', ['watch']);

    grunt.initConfig {
        watch: {
            css: {
                files: ['public/css/src/*.less',],
                tasks: [ 'less', 'cssmin', 'clean' ],
            },
            options: {
                forever:true,
                livereload:true,
            }
        },
        clean: {
            css: {
                src: [ 'public/css/src/*.css']
            }
        },
        less: {
            compile:{
                files: {
                    'public/css/src/style.css': ['public/css/src/style.less']
                }
            }
        },
        cssmin: {
            dist: {
                files: [
                    {
                        'src': 'public/css/src/style.css',
                        'dest': 'public/css/style.min.css'
                    }
                ]
            }
        }
    }

package.json

{
	"name": "El-Nombre-de-Mi-Proyecto",
	"version": "x.x.x",
	"devDependencies": {
		"grunt": "~0.4.2",
		"grunt-contrib-watch": "",
		"grunt-yui-compressor": "",
		"grunt-contrib-less": "",
		"grunt-contrib-clean": ""
	}
}