Arrange photos by filename
You can use the "Auto sort" option in customize gallery, or you can use one of the photo tools to do a manual quick sort by filename. Regardless, the way you number your files before you add them to SmugMug is important.
Computers sort by looking at the first character/digit, then the second, then the third, etc. They don't see a three digit number as a whole.
Example: IMG_100.jpg will be before IMG_23.jpg . The computer sees that 1 comes before 2 and then stops (rather than seeing that 23 comes before 100).
So when you name files on your computer, you might consider doing it like this: 00001.jpg 00002.jpg .... 00010.jpg ....000123.jpg , etc.
In the example, above: IMG_023.jpg will be before IMG_100.jpg because the system will recognize that 0 comes before 1 .
Arrange photos by date taken
Be sure to avoid the "Save for Web" option in programs like Photoshop, if you'd like to arrange photos by date taken. That option will strip the photo from all the extra info that is usually packaged with it. It'll reduce the file size, but you won't be able to easily arrange your photos by date.