They all do it. =) Spock started months ago. Also only recently meant no itms.
We've always treated her no with respect. Eg "no tickling!" We stop. "No bread!" We dont give her bread with tea. That way she learns the consequences, and often will then think and want the item, eg ahe sees us eating the bread she then asks for some. We explain she said no and thats why she didnt get it etc... weve found this helps her take our no seriously too, so when we do say no to her ahe stops and thinks about what she is doing.
I also always give her 2 options, mainly when getting dressed, ill pull out 2 pants and say 'blue pants or brown pants?" Some days she will still say 'no pants!' But that is rare, and then iwait 5 mins (if theres time) and try again explaining we need to get dressed so we can...what ever we are doing