This is a tip I had almost forgotten about, until I resized down a simple GIF image, and remembered why it had come out such bad quality!
When you open a GIF in Photoshop, it opens it in Indexed colour, not RGB. This is not surprising, as GIF image compression works by reducing the number of colours down to reduce file size. However, the real trouble starts when you want to resize that image - the quality is awful!
Remember to put your image back into the full RGB colourspace before you resize it, it really will make the world of difference! Obviously, resizing image up always reduces quality, but if you are a web developer, often you just need to reduce down the size of a GIF a bit to make it fit in, and can’t put your hands on the last full uncompressed editable file!