Although this is a little off topic for how and what to negotiate, I think it is worth some time to ask ourselves, “Why don’t we ask for more?” Seeing how well my daughter innately negotiates, I think the reason is that society trains women that it is improper to negotiate or even to ask for what you really want and need. We are trained to be quiet, good, and to make do with what we are given. Sometimes, this is definitely called for. Sometimes, you need to make do. But, doing this too often makes us complacent and gets us out of practice for negotiating when we need to.
This reminds me also of self-promotion. It is also improper to self-promote. It is trained out of us. But, like negotiating, it is essential to success in academic science. Perhaps also it seems a bit mean to self-promote, but propping yourself up is not synonymous with putting others down. In fact, self-promotion and good negotiating can have positive impacts on your department (more money to go around, more prestige, higher impact, better students).
When we are feeling weird about these things, try to remember the positive impacts they have not only for you, but for the people in your lab and department. By putting these actions into positive context with others, we can break through the self-imposed barrier on these crucial skills.