SmugMug?'s Proof and Retouch feature is an extremely powerful tool for pros.  Essentially, you upload smallish, unretouched proofs to a gallery.  The customers buy, and you have from 1 to 7 days (your choice) to retouch and replace the photos.  Not only that, but you can also make adjustments to the customer's chosen crop, as well as the True or Auto Color choice.

First, plan ahead.  Some pros prefer to shoot RAW + small JPG at shoot.  Great, you can just upload the small in-camera JPGs.  Other pros shoot RAW only, and use the RAW conversion software to generate the proofs.   Still others may shoot only JPG, full rez in-camera, and batch resize these prior to uploading.  Any of these methods will do.  Use what's best for you.

For proofs, you can upload your files fairly well smashed.  Here's a gallery with some examples from three Canon Cameras, at four different resolutions.  These files were all converted from RAW, at different sizes. Of course, this is all easy to do in batch mode in Photoshop if you do not shoot RAW+JPG.   Need help with that?  All the photos were saved at Photoshop JPG-7.  Yes,  JPG-7 will display these types of photos just fine, for sale.  Of course, if you have fewer files, or don't mind uploading a bit larger files, then compress them at JPG-8.   Here are some examples of how much the files can be compressed:

1Ds Mark II, 16 megapixel file, 1.2Mb
1Ds Mark II, 6 megapixel file, 332kb
1Ds Mark II, 3 megapixel file, 540kb
1Ds Mark II, 1.5 megapixel file, 252kb

5D, 12 megapixel file, 1.3mb
5D, 6 megapixel file, 764kb
5D, 3 megapixel file, 440kb
5D, 1.5megapixel file, 312kb

20D, 8 megapixel file, 1Mb
20D, 6 megapixel file, 828kb
20D, 3 megapixel file, 484kb
20D, 1.5 megapixel file, 348kb

Update: Adobe Lightroom Compression examples:

You can see some examples here in this gallery - using compression in Lightroom at even level 30, produces a fine image for display purposes.  Of course, it's up to you to decide how much - but you can export from Lightroom, at compression 50, and hoave quite manageable file size for uploading.  The compression levels and file sizes are in the captions of each photo.

Be very careful to upload files with resolution great enough to support the largest size print you wish to sell.  If you don't, the sizes won't appear in your shopping cart.

NOW, with the new ability in your pro sales page, your customers can buy, and crop or not crop as they choose. Upon notification of order (you get an email from us when the customer orders), you, go into your control panel during the proof-delay period, view the order. You can reupload exact same file size and resolution, with color changes. Or, you can also reupload a new photo with much higher resolution if you wish. You must then see the new crop lines on the newly replaced photo, the crop would be off, so you MUST go to "adjust crop" and crop back to taste.

The final step in the process is to hit the "Ship It!" button, and the order will now go off to our lab.

Here is a sample real-live gallery in action.