One important unwritten rule, of course, is start reaching out now to potential advisors. The things that will make you stand out in a crowded field (even now) are all the more important if your advisor knows to be looking for you. Your application, if strong enough for the program as a whole, will cross their desk and they will be asked to opine about it. That's when we fight to get certain students. It's also a good way to find out if your potential advisor is a good fit, or if there are problems at the institution that should warn you away (for example, at Alabama a potential advisor gave me a lot of detail about problems there that eventually led me to skip applying; I got a really egotistical response from Toronto; silence from Maryland-College Park; and a welcoming and lengthy discussion and invitation to come visit from my eventual graduate school). You don't say where you're completing or where you're aiming, but that's true just about anywhere. It's very helpful to be someone an advisor wants to work with, so that does require that you be at least in communication with their work. A good potential advisor will also put you in touch with other graduate students, and give you advice on situating your application for best effect. (A good one will also be honest with you, if you are being unrealistic in your aspirations--some of us had to go elsewhere for an MA first, and then climb the ladder again.)

Second, and this depends on data we don't have from you about your potential fields, institutions, etc., if you can't get into a top-15 program for your particular area of study, with funding, you should consider alternate ambitions or acquiring additional qualifications that will stand out. The market is terrible and very, very few make it straight through. Many more make it from top-tier schools (BA to grad and grad to job market) but not all. Graduate study is isolating, it can be alienating, and it will eat up your crucial 20s. Be prepared for that non-monetary investment, and avoid compounding that with debt. We don't make a lot of money out here.

If the latter paragraph isn't prohibitive, then consider the former. It really is "who you know" in this crowded field--but you can make a lot of that contact yourself.

Sorry if this seems terribly focused in one particular direction, but I've been counseling potential advisees for Fall 2016 just this last week, and talking to outgoing history majors as well.

[edit: oops, semicolon loss]

1. I don't what you mean by "supervisor." Is this an American school you're at? In the US, "supervisors" or, more commonly, "advisors" are only at the graduate level. At any rate, what matters most is your work not someone else's. So your writing sample, personal statement, and transcripts. The letters only confirm or further elaborate those documents.

2. It's fine to be open-minded to topics but graduate programs, at least the top ones, prefer applicants who actually know what they are doing and know what they want to do. The proposal you raise is a reasonable one. You should do that anyways because you're actually applying to work with a PERSON not to be in a PROGRAM. If there is no one working on tree-trimming studies and that's one of your interests, then it would be pointless to apply there.

3. It would be nice to contact potential advisors beforehand to gauge their interest and willingness to accept students. Don't send them long inappropriate emails. Do lots of research on them and the program. Ask pointed questions not general, lazy questions. There's templates out there on how to write such letters to faculty.

1. It depends on the specificity of your research. I study WWI under the umbrella of environmental military history. So for my thesis committee I have one environmental historian, one WWI historian, and a modern art historian. Together they cover my whole thesis but not individually.

2. If you have two topics that cannot be combined, research which one has more primary sources and perhaps by going over them you may be able to choose! What are they? I was able to pull all my interests together but that isn't always possible.

3. Be polite but firm when debating schools of thought. No need to be an ass but you also don't want to be a pushover. Never take your thesis coordinator or a Graduate assistantship for granted. I have seen that road taken by a couple of my now former peers and it isn't pretty. When applying for graduate programs be they masters or doctorate, look at the turnout rate, compatability of topic and personality, and the age of the proff you are working with. No one wants their mentor to die or retire before you finish and finding another to replace them may prove difficult.

EDIT: words

Are you thinking about getting an MA first or going straight into a phd program?