All the advice here is really great, but I'll throw in my two cents anyway.
As far as theory goes, I wouldn't worry too much about it unless you're thinking of pursuing it. Getting an idea of the different schools of criticism/theory (20th c.) is probably most helpful. What is really useful about a lot of theory reading is that you develop a methodology of your own, and being able to articulate that in an SOP and show that in a writing sample is always a plus. Additionally, it will help you when you start looking at schools as you'll be able to determine how that faculty looks at and understands literature (psychoanalytic vs. formalist vs. new historicism, etc.) and see if it matches well with your own approach. Being able to parce fine distinctions between theorists is less important, I think, than understanding on a broad level how schools of theory differ.
Talk to your professors. All the time. And let them know you're interested in graduate study. Pick their brains about schools, their field, their methodology, etc. Ask them about themselves and their opinions more so than "What should I do to prep for grad school?" However, if you become close to a professor on an adcomm, they can be a really useful resource when you go to apply if they're willing to look at your writing sample and SOP. But just generally speaking to profs lets them know that you're looking at grad school and you may eventually come to them to ask for a rec letter. I took repeat classes with professors I really respected and they wrote 2 of my 3 rec letters. I felt really confident having them write them on my behalf because I knew they were very familiar with my academic work and goals.
Absolutely do an honors thesis/independent research project. This will help you solidify your interests and give you experience doing independent research (and it provides a great writing sample, especially if its been polished and read by a number of professors and peers). On a similar note, and echoing what someone said above, take a broad range of English courses, even if you already think you know what you're interested in. When I was a sophomore I wanted to do 18th c./restoration lit, but the more courses I took, especially as a junior when I started writing really substantial essays, I settled on the 19th c. And going back to my previous point about profs... I took two courses with the prof who is now my thesis advisor and who also wrote one of my rec letters. She has helped me immensely, from advising me on how to revise my SOP and writing sample to compiling a list of schools when I started applying (she is also in 19th c. studies, so she could recommend great faculty members for me to work with, too).
But mostly, just keep doing what you're already doing. Keep challenging yourself inside and outside of the classroom, talk to lots of people and get as much advice as you can, and hone your interests.