Clarify your goals.
Be clear about what your goal is -- whether you need data from a co-worker who is ignoring your request, want to be director of operations in five years, or want your husband and kids to help with household chores. Whatever it is, you need to state what you want clearly. Any salesperson will tell you they hear a lot of nos on the way to a yes. Ask questions about the issue, be clear about why you're standing up for yourself, and give reasons that support your position. Be realistic and prepared to negotiate. You may not get what you originally asked for; in fact, you may get something better.
Cultivate the right attitude.
Be your best self. We've all had moments when we felt powerful and in control -- maybe when you first rode your bike without training wheels, or when you aced the interview that got you hired. Think back to that experience and remember it in detail. Feel it in your body, your posture and your breathing. Learn to call it up and duplicate it when you need it. It may feel forced at first, but with practice, it will begin to feel natural. Attitude is everything. Coming from the stance that you are a powerful and valuable person can change the whole conversation. It doesn't mean you'll get everything you want. It does mean you won't be kicking yourself for not speaking up.
Don't reinvent the wheel. Look for role models who have the traits and skills you lack. Watch them and learn. Find a mentor or coach. And remember: Each time you stand up for yourself makes the next time easier.