When you have done a superb job and have the opportunity to ask for repeat business, or a referral or a new contract, seize the moment!
If you let it slip, it will be very hard to rekindle that moment.
When the timing is right, do not procrastinate. In fact, do not wait for the perfect time because there is no perfect time. The perfect time is when you know you have a very good chance of success and little to lose.