I'm German.
thumbwitch: LOL. Me too. Was told I was 'blokish'.
It's my profession which is mostly male dominated that allowed me to develop my assertiveness (to certain extent read agression). I am very intuitive and careful about who I am talking to in which way. I say it nicely once and might do it a second time round. Then it comes down to who I am dealing with and if I have to maintain a relationship with them. If they are subordinates and/or people I might not come in close contact with again and want nothing from, then the gloves come off and I'm being very clear and concise in phrasing what I expect from the and where the boundaries are. Always phrase your request in a way that focuses on the task at hand, not the person. This way you aren't really challenging him/her but describing the issue only.
If you're dealing with the likes of partner, family, friends, colleagues, etc. then the best course of action is to a) sound non-accusatory, b) do not use words like "always", "never", any black and white phrases, you get the drift. Make your request sound very neutral and ensure your voice isn't giving your nervousness away. Tone and slow precise speech will make you sound a lot more assertive and alpha, then what you say IYSWIM. Focus on the issue at hand, negate e.g. taking the kid out for MacD by saying: I'm afraid I have already made other plans for that WE. Or: Regrettably I am not available that evening, day, etc. I agree with others, be vague, do not commit yourself to anything. Say you need to check and revert. You cannot agree now.
Practice in shops, etc. with strangers, etc. say no, disagree. Go over convos in your head and have a few phrases at the ready.
Will check and revert.
Dont have my diary with me.
I'm afraid I already have plans for that day, evening, etc. (If asked what, then you can either say: You're not nosy, huh? or: It's private.)
Please make sure x y z is done in x y z way.
I'd really appreciate if you could do x y z next time you do it.
yada yada
Also: keep your tone confident as I said before and keep your eyes steadily on the person you're speaking to. Try it. You'll come across a lot more confident. We all communicate much more with our voice and body than with the mere words we utter.