Grunt: Compass, Autoprefixer, CSS/JS minification and Livereload. Use the `npm install`and `grunt` commands consecutively to execute.

Gruntfile.js

module.exports = function(grunt) {

	grunt.loadNpmTasks('grunt-contrib-compass');
	grunt.loadNpmTasks('grunt-autoprefixer');
	grunt.loadNpmTasks('grunt-contrib-cssmin');
	grunt.loadNpmTasks('grunt-contrib-uglify');
	grunt.loadNpmTasks('grunt-contrib-watch');

	grunt.initConfig({

		// Reference package.json
		pkg: grunt.file.readJSON('package.json'),

		// Compass
		compass: {
			dist: {
				options: {config: 'config.rb'},
			},
		},

		// Auto Prefixer
		autoprefixer: {
			dist: {
				options: {
					browsers: ['last 1 version', '> 1%', 'ie 8']
				},
				files: {
					'css/style-prefixed.css': ['css/style.css']
				}
			}
		},

		// Minify CSS
		cssmin: {
			combine: {
				files: {
					'css/style-min.css': ['css/style-prefixed.css']
				},
			},
		},

		// Minify JS
		uglify: {
			my_target: {
				files: {
					'js/scripts-min.js': ['js/src/theme.js']
				},
			},
		},

		// Watch
		watch: {
			compass: {
				files: 'scss/**/*.scss',
				tasks: ['compass'],
			},

			csspostprocess: {
				files: 'css/style.css',
				tasks: ['autoprefixer', 'cssmin'],
			},

			jsminify: {
				files: 'js/src/*.js',
				tasks: ['uglify'],
			},

			livereload: {
				options: {livereload: true},
				files: ['css/*.css', 'js/*.js', '*.html', 'img/*'],
			},
		},

	});

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

package.json

{
  "name": "projectname",
  "devDependencies": {
    "grunt": "~0.4.1",
    "grunt-contrib-jshint": "~0.6.0",
    "grunt-contrib-compass": "~0.5.0",
    "grunt-contrib-watch": "~0.5.2",
    "grunt-contrib-cssmin": "~0.6.1",
    "grunt-autoprefixer": "~0.2.20130806",
    "grunt-contrib-uglify": "~0.2.2"
  }
}

Where's the csspostprocess task :)