Hi Alyssa,
I'm probably the last one who should give advice on this, as I am not able to dress my gender yet. It's a job/location thing and it kind of sucks. Anyway, there are some good points here regarding presenting as male sometimes and female other times. I'm sure that doesn't help. Barbie gave good advice, simply tell them what you prefer. Your response that you are "having trouble getting to tell them" is, well, a cop out. I'm sorry, I don't mean to be rude or anything, honestly. It seems like the the time to tell them is when they use the wrong pronoun. Right at that moment. That is your chance.
Like I said, I'm not where you are, so I'm learning from your question and the responses that I see here. I just hope that after my response to you, I have the sense to follow that same advice. Again, my apologies, I don't mean to be rude.
I meant to add also that I'll have to remember the response from Jessie. I can see that working, too. Janet, I'll have to wait until I get noticeable boobs!