Create icon pack for iOS or Android devices. Recommended source image size is 1024x1024 px 72 dp, make sure visible layer is selected. Workflow: source image is flattened, bicubic resize applied for each size, results saved in /output folder in source directory. Best practice: create photoshop Action with script, bind it to some hotkey.

ps_build_icons.js

// Created by Roman Baulin on 17.05.2012

#target photoshop

// in case we double clicked the file
app.bringToFront(); 
 
var doc = app.activeDocument;
var startState = doc.activeHistoryState; // save for undo
var initialPrefs = app.preferences.rulerUnits; // will restore at end
app.preferences.rulerUnits = Units.PIXELS;     // use pixels

main();

function main () {
  // Save icons in PNG using Save for Web.
  var sfw = new ExportOptionsSaveForWeb();
  sfw.format = SaveDocumentType.PNG;
  sfw.PNG8 = false; // use PNG-24
  sfw.transparency = true;
  doc.info = null;  // delete metadata
  
    var iconsIOS = [
    {"name": "Icon-Settings-29",   	  "size":29},		// spotlight
    {"name": "Icon-Settings-29@2x",	  "size":58},		// spotlight retina
    {"name": "Icon-Spotlight-40",       "size":40},   // spotlight ipad iOS 7
    {"name": "Icon-Spotlight-40@2x",    "size":80},   // spotlight ipad iOS 7
    {"name": "Icon-50", 	    "size":50},		// spotlight ipad	
    {"name": "Icon-50@2x",    "size":100},     // spotlight ipad retina
    {"name": "Icon-57",       "size":57},   // iphone
    {"name": "Icon-57@2x",    "size":114},  // iphone retina
    {"name": "Icon-60@2x",    "size":120},  // iphone retina iOS 7
    {"name": "Icon-72",       "size":72},   // ipad
    {"name": "Icon-72@2x",    "size":144},  // ipad retina
    {"name": "Icon-76",       "size":76},    // ipad iOS 7
    {"name": "Icon-76@2x",    "size":152},   // ipad retina iOS 7
    {"name": "icon-512",      "size":512}, // appstore
    {"name": "icon-512@2x",   "size":1024} // appstore retina
  ];
  
  var iconsDROID = [	
	{"name": "drawable-ldpi/ic_launcher",   "size":36},
	{"name": "drawable-mdpi/ic_launcher",   "size":48},
	{"name": "drawable-hdpi/ic_launcher",   "size":72},
	{"name": "drawable-xhdpi/ic_launcher",  "size":96},
	{"name": "market/ic_launcher",  		"size":512}
  ];
  
  
  if (!createFolder(doc.path + "/output/")) {
    alert("unable to create output folder");
    return;
  }
  
  var ui = // dialog resource object [left, top, right, bottom]
	"dialog { \
        text:'Generate icons',bounds:[0,0,210,50],\
		alignChildren: 'fill', \
			orientation: 'row', alignment: 'right', \
			iosBtn: Button { bounds:[10,10,100,40] , text:'iOS' }, \
			androidBtn: Button { bounds:[110,10,200,40], text:'Android' } \
	}";

	var win = new Window (ui);
    win.center();
    win.iosBtn.onClick = function() {
        win.close();
        createIcons(iconsIOS, sfw, "iphone");
        return;
       }

    win.androidBtn.onClick = function() {
        win.close();
        createIcons(iconsDROID, sfw, "android");
        return;
        }
    win.show();
 }
  
 
 function createIcons(icons, sfw, device) {
    if (device == "android") { // on Android we need to add spacing
        expand = 16;
        bounds = new Array(-expand,-expand, 512+expand, 512+expand);
        doc.crop(bounds);
    }
 
  // if mask is selected it wont work
  doc.mergeVisibleLayers();
    
  var mergedState = doc.activeHistoryState; // save for undo
  
  var icon;
  for (i = 0; i < icons.length; i++) {
    icon = icons[i];

    doc.resizeImage(icon.size, icon.size, null, ResampleMethod.BICUBICSHARPER);

    pathElements = icon.name.split('/');
    if (pathElements.length > 1) {
          if (!createFolder(doc.path +"/output/" + pathElements[0] + "/")) {
            alert("unable to create " + pathElements[0]);
            return false;
          }
    }
    
    doc.exportDocument(new File(doc.path + "/output/"  + icon.name + ".png"), ExportType.SAVEFORWEB, sfw);
    doc.activeHistoryState = mergedState; // undo resize
  }
  app.preferences.rulerUnits = initialPrefs; // restore prefs    
  doc.activeHistoryState = startState; // undo resize
    return true;
}

function createFolder(fptr) {
if (!fptr) {
  Error.runtimeError(19, "fptr");  // Bad Argument
}

if (fptr.constructor == String) {
  fptr = Folder(fptr);
}

if (fptr instanceof File) {
  return createFolder(fptr.parent);
}
if (fptr.exists) {
  return true;
}
if (fptr.parent && !fptr.parent.exists) {
  if (!createFolder(fptr.parent)) {
    return false;
  }
}
return fptr.create();
}