salary negotiating

I have been terrible at negotiating rates. I haven't been great at asking for raises. For years I made $10/hour as a pseudo-manager at my retail job, and found out much later that everybody in the same position was making at least three dollars more than me. Even now, when asked how much my rates are as a writer, I'm tempted to lowball myself, convinced I'll come across as greedy or unappreciative if I toss out an amount that seems too high.