I just randomly remembered this incident I found rather awesome in which I was queueing in a McDonald's and this lady pushed in front of me about ten seconds before I got served. The team member or whatever the insist on being called went to make eye contact with her and ask her order, when I clicked my fingers in front of his face and said "Mate, serve me first; she pushed in front of me. I'll have..." and didn't even give them a moment to react.
He took my order without any hesitation (I must've been scowling something fierce) and the lady then turned to have a go at me, at which point I lectured her on proper queueing and how she, being visibly older than me, should've grasped that. She immediately dropped it.
Oddly enough I am never this assertive the rest of the time (internet conversation notwithstanding) other than around friends who are willing to tolerate it (I'm fairly sure they know I'll take as much from them... oh god I hope so anyway) but I honestly wish I could be.
Of course I think there's a line between assertive and just being a dick, and I have trouble defining it, which is why I often don't do anything for fear of the times I've crossed that line unwittingly.
How assertive are you guys capable of being with different kinds of people?