First - any time you write a terms/conditions/agreement document, consider where you will get your legal advice (always good to do, just in case).
Either way, legal advice or not, here are some that have worked for me in the past:
Reserve the right to reject any advertisement at any time. This is a catch-all safety net for you to deal with complaints, quality, etc
Similar to #1 above, reserve the right to cancel any advertising run (e.g. to cancel a 12-month agreement on month 3 if you want)
Decide what you will always reject. Be clear about it (e.g. profanity, adult/pornographic, drug-related, alcohol, gambling, etc., cigarettes, etc.)
Be clear on your rates and payment terms (e.g. payment in advance, payment methods, how they ask for a refund, do you offer refunds at all, etc)
Clearly state what you don't gaurantee. E.g. if you sell per-impression, state you can't gaurantee clicks. If you sell clicks, state you can't gaurantee impressions or conversions etc.
Be clear on where the ads will show and reserve the right to change this at any time in their agreement. We once had a client argue because we tweaked the design of the website, and a 10px shift in their banner location was a big issue to them. This seems silly, but we didn't properly guard ourselves on this one and ended up losing the advertiser.
Reserve the right to change the rates (e.g. if you bill them $200/month, and need to increase it to $220) but promise a notification window (e.g. 30 days in advance)
Be clear on what reporting and analytics you provide and don't provide.
Reference a style guide and in the guide discuss the elements like file format, file size, location, content guides, etc. Let this document be more flexible - the contents don't need to be enshrined in the Terms and Conditions.
Good luck!
