The primary purpose of terms and conditions is that the client understands what they are and that no misunderstandings arise.
Not sure what you mean by a "web agency" but I assume it's some sort of service to a limited number of clients as distinct from a massive number of people buying something from your website?
Therefore, I think you should write a letter/email to each new client in a friendly manner thanking them for the business and attaching the terms and conditions and maybe outlining some of them in the email. Pick one or two which have given rise to problems in the past e.g. "The quotation of €1,000 is for the initial design according to the spec provided by you and some reasonable amendments based on that design. However, if you change the spec. materially, we will charge you for additional time at €50 per hour."
Brendan