A girl has openly told me that I need to be more assertive. She is also difficult to hang out with because I think I give her too many options ("when will I see you again?" and other stupid beta shit). Is the next move to counter this something along the lines of calling her up and saying something along the lines of, "listen, I'm coming over?"

When a girl tells me something like that I will respond in an over the top manner to basically punish her for saying that, and to shut her up. I don't mean that I'll actually hurt her. But if a girl says you are not assertive enough or too passive, grab her push against the wall and man handle her, and be an aggressive asshole. That may not be exactly what she wanted you to do, but I doubt she'll go around saying you aren't assertive enough!

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“Nothing is impossible, the word itself says 'I'm possible'!” ~Audrey Hepburn

She is telling you that you have to stop asking question and start making statements. I remember this one girl who gave me this fluff talk about "Oh, I might be busy" or "I will consider it" and all this bullshit. I got fed up with it and just assumed she was coming over so I would send her text the day of like, "wear something comfortable" or "I better see a smile on that face later". Believe in yourself and assume the best. Why do you think my name is what it is...

Assertive and aggressive are not the same thing. According to http://en.wikipedia.org/wiki/Assertiveness , assertiveness is "a form of behavior characterized by a confident declaration or affirmation of a statement without need of proof; this affirms the person's rights or point of view without either aggressively threatening the rights of another (assuming a position of dominance) or submissively permitting another to ignore or deny one's rights or point of view. ... Aggressive people do not respect the personal boundaries of others and thus are liable to harm others while trying to influence them." So take care not to go from non-assertive all the way to aggressive, balance is the key. Since assertiveness is situationally dependent, Puaninja's suggestion may be assertive at the right time, but come off as very aggressive at other times.

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One of the most useful things you will ever learn about body language.