Read read read read read. Courses you plan on taking - do some reading about the area beforehand. If you have a syllabus with planned discussion topics, read ahead on those to be familiar with what the teacher is talking about. Bring up any questions you may have - perhaps you will discover something important that isn't covered. I am a history major in my junior year and from my experience, not a lot of students speak up or give input. It's often as if they don't do the reading and are just there for the credits, which I am sure is mostly the case. Not only Impressing but also building a relationship with a professor can be very rewarding especially in the history/teaching field.

Find what you love and be open to what you don't. Despite being a Francophile, I took a class on Late Imperial China, and I kind of liked it.

Don't expect college history to be like high school history. At least where I'm from, college expects you to have the historical background in mind by yourself. You won't be taught the grand schemes of History anymore, so, as /u/jamezdee said, read read read read and read. You may want to switch between books (textbooks and monographs) and shorter articles (you'll love JSTOR) every now and then to avoid an unpleasant reading routine. If you have a Kindle or iPad or sth like that, put some new articles on it every once in a while, it makes boring moments much better to read something interesting that advances your knowledge.

Learn to read quickly. Some things you will have to take your time with, other things you just need to get the author's major argument and some of their supporting evidence. Figuring out what sources you need to read quickly and which ones you need to take your time on is another skill.

Also, if you're a historian you should be a linguist. Learn as many languages as you can to the absolute best of your ability. Personally, I screwed up this step and my language skills aren't up to snuff. Don't repeat my mistake. Pick up two or three languages as an undergrad if you can--consider summer school or private study and taking tests to speed things along. Don't just take the one required language--push yourself here.

Finally, there's an adage I only heard after getting out of college, but I think it's quite clever. Sleep more than you study, study more than you party, and party as much as you can. If you keep sleep, study, and partying/socializing in that ratio you will do fine regardless of what you're major is.

I had a system of revising for exams that I found quite helpful but your mileage may vary. I can't stand re-reading my old notes, it's really boring, so I tried to minimize that as much as possible. What I would do instead was buy a general academic history on the subject (for example when I finished my Crusades course Thomas Asbridge's book on the subject had just come out) and read through the relevant sections from that book. What I could do is look at the work with a critical eye, check their references, identify when they were describing vs. when they were interpreting the evidence, and then see if I agree with their version of events. I found it to be a lot more interesting. If you really want to work at it you can pick a much more controversial history book as well, someone with a well known bias, and work your way through that.

Be very open to new experiences, take papers that lie outside your immediate interest areas - there are some fascinating topics taught by excellent lecturers that you'll otherwise miss out on. Talk to senior students and find out which classes they really enjoyed and found interesting.

Learn how to quickly read a secondary source when doing research, particularly how to identify thesis statements. This is very important when you are going through massive numbers of texts for your assignments, since you probably won't have time to read every source. It may also come in useful if you, for entirely legitimate reasons beyond your control, were unable to finish the prescribed readings. One suggestions is for a journal article, read the first and last sentence of every paragraph, as well as the entirety of the opening and closing paragraph.

Good research technique in general is essential in order to use your time efficiently and produce great results. If you find a useful book, read its bibliography - chances are there'll be something useful in there as well. Keep a record (digital or handwritten) of the sources you read, and a detailed list of any useful information you find so that you can easily refer to it and cite it in future. My preferred technique is to write a document with small to medium quotes or summaries, with citations, for each major source. Then I work out the loose structure of my essay (or literature review etc.) and combine my source nuggets into one large document according to how they will fit into the structure. I then write the essay, utilising the relevant quotes and citing them as I go along, and every time I use one I significantly reduce its font size in the other document so that I can see that I've covered that point. Ideally, at the end I'll have one well-structured, well-supported essay with citations.

Attend optional seminars/presentations/lectures. These are platforms for historians to present their research and reveal their passions, and lets you build contacts/impressions for the future. If there's a recommended/supplementary reading section in your course syllabus, take a look.

And at all costs avoid plagiarism and work on smoothly including paraphrasing and quotations into your work (with citations, of course). Being able to work citations into closed book examinations is worth a lot of marks.