SonieTheDog has some super good points! I few I would add are...
-Edit guidelines during the art process. How many free edits and how large then can be before a fee will be applied. This helps stop those who ask you to keep changing details (not because they are wrong, just because they thought of something new or wanna add stuff) that were not discussed before hand or added to how much it would take and thus the price.
-Contact info. In case they need to get a hold you of outside of DA or wherever in case the site goes down or someone gets too busy. Having a separate email just for commissions is a good idea for this.
-Useful links. Sonie mentioned the queue and it's good to have a direct link to the queue in case someone needs to find it. Linking to your FAQ, other art sites, etc are good too.
-If you do adopts you can add in an adopt only section for them. It would help cover some things a general TOS might not. Of course if the TOS is about adopts and not commissions then it would be a moot point.
-Delivery policy. How will you be giving them their files? Dropbox, GoogleDrive, DA stash? Will you be giving them a watermarked file for web posting and a large file?
-Posting. Going off of the delivery policy. Can the client post what they have gotten? Can they only post the 'web' approved version? I know this sounds silly but trust me it's good info. I've had clients ask if it was okay to post the images I've drawn for them multiple times to adding it in can save you some questions.
It dosen't hurt to go around a few other artist's TOS to see what they do, and take notes on what they feel is important to mention and not to see if it applies to you.