I'd say that my biggest pieces of advice, which have also been mentioned above, are:
Apply to MA programs as well if you can. Or have some sort of plan B. Before I received acceptances I was very stressed because I didn't have any other plans.
Get started early! I did this and it made the process much less overwhelming.
Having a professor read over my SOP and answer my questions about grad school in general was such a great help. Definitely try to find a mentor.
Reach out to POIs before you apply. It's nice to know if they are accepting grad students, and to see what they're really like. This saved me from sending in a few applications that probably would've been tossed since my POI was about to retire.