Whoever says happiness comes from reality... is wrong. For example, If I tell you I'm going to give you $50 and only give you $20, you'll probably be disappointed. But if I promise you $5 and give you $20, you'll be overjoyed. Same reality, different levels of happiness.
The same is true at work. Always under-promise and over-deliver. Whatever the reality, your clients and co-workers will be much happier if they expect less.
So that's probably nothing new, but how does it translate to marketing and graphic design? You can't promise a good website and deliver a great one because it's all subjective. So before you begin a project, think of all the ways to objectify its results. Then over-deliver.
Promise 3 drafts of a website layout, then present 5. Promise to spend no more than 30 hours on a project, then spend 50. Promise to charge no more than $1000, then factor in a reduced price on the invoice. When you objectify the results, it's much easier to over-deliver. And when you over-deliver, you're more likely to get happier clients, which equals more referrals and future work.