Be positive. Parents often forget that a toddler's social behavior is motivated not only by a desire to avoid displeasing others (especially parents), but also by a genuine desire to please. All too often, parents offer behavioral guidelines only in negative terms. If you take the time to praise good behavior often enough, your child will eventually make an effort to repeat it.
Like all toddlers, your child needs to be allowed to explore her environment. This exploration is part of her growing independence, confidence, and separation from you. (Don't worry, your child won't separate too much for many more years to come.) If you want your toddler to make the most of her early adventures, you'll need not only to provide her with the opportunity to make new discoveries, but actually to encourage exploration and experimentation.
Constantly having to tell your toddler "No!" will do exactly the opposite. It pointedly discourages your child from exploring her environment-at least parts of it.
In addition, the sternness and suddenness with which you say, "No!" will probably frighten your child. Many toddlers burst into tears, falling apart whenever their parents say, "No!" In general, your toddler doesn't like to do anything that displeases you. Oh, your child certainly has a will of her own. And when push comes to shove, your toddler would much rather get what she wants than sacrifice it for the sake of avoiding your displeasure. Nonetheless, conflict with you, a clash between your desires (for her safety, for example) and hers (for free reign) is very scary for your toddler. It feels dangerous to displease you. In your child's mind, the thought of your disapproval is equated with rejection, and therefore intensifies any abandonment fears.
So try to avoid saying, "No!" all of the time. Whenever you do say it, follow up by comforting your child. Explain in concrete terms why you wanted your toddler to stop doing what she was doing (danger to herself, danger or harm to others, and so on). Above all, emphasize that even when you get angry at her, you still love your child. Toddlers, so richly anchored in the present, often have a hard time realizing this.