Well, it can be tricky sometimes. For the most part, my time and stuff is built into my contract.
But if they are asking for small things, I generally just do them, as it fosters a good relationship.
If/when they start asking for MORE and MORE or too much or too big of things, I simply email them to let them know that “thing will take longer than we agreed on and I don’t have the time for this right now. If you are interested in having me do this now or on an ongoing basis, let’s jump on the phone and renegotiate my fees. Thanks!”
And that is a polite but specific response which works for me.
Most people don’t set out to take advantage, and if it seems that is the type of client they are (pushing back really hard on reasonable pricing and having extreme expectations, etc) during the initial process, I will make the decision NOT to work with them before we get into everything. That’s why I ALWAYS do phone calls and try to get to know a client before we start working together. Here is an article I wrote about this exact topic!