If you are creating masks (black/white or gray scale images), a simple way to pack masks is to take the mask and copy/paste it into one of the channels of a new image (RGBA). Once taken into an engine you can use whichever channel you need to drive a mask for the desired effect. This is an excellent method to have many masks available while keeping the performance impact low in a real time environment. To do this:

1. Create a new photoshop file with the texture size you want;
2. Fill the first layer of the new file with either black or white (if you miss this step, the copy/paste will not work);
3. Convert your grayscale images to RGB;
4. Copy and Paste your grayscale maps as layers in the new texture file;
5. Double click a layer and untick the channels that the map should not be. This works because a grayscale image is the same in all channels.

*Note: You can get some issues with 8 bits/channel depending on your render engine.