Good employees value themselves. Asking for what you are worth will not be seen as a negative. Rather, some potential employers even look favorably on people who negotiate appropriately. It shows initiative, confidence, and that you have an understanding of the value you expect to bring to the company. Compare that to someone who interviews and then just jumps at the first offer thrown at them... it shows lack of confidence and initiative. No person in the history of ever has ever lost a reasonable job offer because they came back with a higher salary request.
That said, you need to know how to 'negotiate' correctly. It isn't some high stakes game of chicken, and that's what people get wrong (and that's why people are scared to play). Simply know your market value, know what you need to take the job, and have a few arguments or pieces of evidence that show you are worth what you are asking. After that it's not a game. Simply say something along the lines of:
"I'm very excited to receive the offer and would love to come work with you and your company but you should know I am looking for a salary approximately $25k/year higher than what you quoted. Given the role you are offering and the value I plan to bring to the company, I feel that's a very reasonable request given my experience, my current salary, and what is available to people of my level of experience."
The only time you can effectively negotiate a higher salary is now, right before you accept the offer. Once you sign, it becomes very hard to push for raises that exceed the corporate schedule.