I just wanted to share how helpful this thread has been. I am really trying to avoid the power struggles now. With my 2.5 yo, I don't make a big deal out of her 'no' to my request. She seems to be learning to exercise this right, and who I am to force my will upon her? Is that how I would care to be treated? Although I admit there is a flash of anger with each 'no', I try not to let her see that. I simply remind her of the consequences of her actions, and gently comfort her when she is angry. There are more fires to put out these days with her, but they fizzle more quickly when she feels not only heard but respected.
With my 3.5 yo, naps were becoming such a struggle. Finally, I told her that it was her choice. She could choose each day whether she felt tired enough for a nap, and I moved nap time back by an hour to accommodate her little sister better. This power of choice helped enormously. She now takes naps because she wants to versus because I told her to. Because I couched this 'deal' in a 'big girl' light, she now helps me with all kinds of daily tasks, like snapping asparagus, or winding balls of yarn, sorting laundry, or wiping down the dinner table. I particularly like to step in with these requests when an argument I cannot solve is brewing between her and her sister - like who was playing with the doll first. I know my older daughter often behaves like a bully with her younger sister, taking away toys and books and generally lording over her, even telling her how to feel (oh,if I had a dime for every time I heard "so and so told me to feel sad!" accompanied with lots of screaming and crying.). I feel like removing her from the situation and giving her some responsibility will help her craft gentleness and kindness toward her sister. (Please don't mistake this for not addressing the issue. We are a no hitting house, and hitting garners an immediate time out and an apology. Smaller offenses involve hearing all sides and issuing apologies.)
So overall, I have learned so much from this thread! So much to work on.