Whats the best way to avoid being cheap while being nice in a professional environment?

What do you do, and what kind of projects are they?

Develop a pre-game. If customers know ahead of time what happens when a project gets delayed/extended they shouldn’t have any trouble paying up when it happens if the reason for the extension is genuine.

You already answered to your own question: the best way is doing your job with the highest quality, passion & the highest professional standards.
This will make your work invaluable, there’s not even need to ask for more money.

Sure you will have to tell them your project needs an extension, but given your results and your reputation, you should face no problems in getting more money.

Defined scope of work, clear expectations, pre-established rates

When you give your estimate, lay out the scope of work and the time you expect it to take. If the project keeps extending, and it’s your fault, you bear the cost; if the project keeps extending because the customer changes his or her mind constantly or because the customer keeps on coming to you with new ideas, say, firmly but fairly, “The scope says you get two comps, and that these are the features that will be included in the finished product. That’s what the estimate is based on. If you want that changed, we’ll need to put a change order in writing and negotiate an additional fee.”

If you set up clear expectations to begin with, it will make it easier when the scope changes. If you have a vague scope, no clear work breakdown, and none of it is in writing, you’re screwed when the customer adjusts his expectations.

Thanks for the answers, very similar pattern of advice: must make things very clear before hand :) All good!