How do you charge for software development services? What is the best way to avoid losses?

If you run your own software development company, how do you charge for your services?

What's the best way to avoid losing money on project delays? I've had instances where clients change their minds and the project is extended, causing us to lose money.

When we inform them of an increase in price, they do not like it and they try to squeeze what they can from the original budget, but causing us to spend more time developing the product.

Any help would be appreciated :)