Also try to keep doubling the size of the image - it makes the resampling easier for photoshop. And in the image > size menu make sure that the resampling rate is set to bicubic.
But as Mike said, resizing a JPEG is going to reveal the problems with the original image. You could try to apply a blur to remove the pixelation and then a sharpen edges filter, but you're still fighting an uphill battle, and no amount of filters is going to replace image information that isnt there in the first place.
good luck anyway