I am a 4th year DO student submitting a (hopefully adequately long) rank list in the coming weeks. You're correct that psych has been more competitive for DO applicants in the recent past. I am curious to see what the next Charting Outcomes shows for us later in 2022. While there is some anti-DO bias, there's also likely a fair bit of self-selection that mitigates this. I imagine there are still a number of applicants who apply to programs they're not competitive for and pay the price. But I also had very similar stats across the board as an MD this cycle and viewing their # and quality of interviews was just mind mindboggling. Didn't feel fair. But oh well, no sense in my dwelling on that right now.
Entering a DO school right now and ~expecting~ any moderately competitive specialty, at this point, is quite risky in my opinion (step 1 going P/F, proliferation of DO schools, persistent MD graduate pressure). Psych seems to be flirting with being "moderately" competitive- it used to be non-competitive as I understand it. I encourage any new DO student to think long and hard about if they would be truly miserable in FM/peds/non-academic IM. If yes, they should re-evaluate the risk of 200k+ debt for a career that makes them miserable. Now onto your question.
I went into med school wanting to do EM. I decided on psych about 2/3 through third year. I deliberately participated in 0 school affiliated extracurricular activities during medical school, to focus on what would get my foot in the door for interviews but also sub internships (AKA Sub-I, acting internship, away rotation): step 1 (for you, it will be step 2). Studying hard for step 1 is probably the best setup you can give yourself for excelling at step 2 though.
Aside from that, how "showing interest" manifested for me was:
Pre-ERAS:
-pursuing psych related research
-pursuing non psych related research (likely not super impactful)
-doing well on 3rd year psych rotation
-applying to sub internships early and thoroughly (complete any "why this program?" questions thoughtfully. Perhaps have someone proofread if you are a poor writer or something).
-reach out to programs with questions and express specific interest
-A PD told me longitudinal psych-related volunteering is beneficial. I didn't have this. They didn't interview me. Maybe start this right after step 2 (do not let it interfere with your step 2 performance).
Intra- and Post-ERAS:
-do well on step 2
-absolutely destroy sub-Is. Show up early. Stay late. Be personable and always in a good mood. Help residents. Psych knowledge is important but it is low on the list of attributes (teachability and interest in learning appear to far eclipse this). Proactively ask for feedback, additional opportunities (<-doing this got me a letter that I was told multiple times by seasoned interviewers is the best letter they have ever read), questions about the program. Express your sincere interest in the city and program.
-attend all social events during interview season. These are soul sucking but I have been told important at certain programs.
-send letters of interest to at least some subset of programs early on in the cycle (these are basically emails saying "I'm great, you're great, please consider my application for interview")
Tl;dr: doing well on step 1 will improve your chances of doing well on step 2. Studying hard for these exams is the most important thing you can do preclinically, for any specialty, including psychiatry. Also discussed clinical activities that would help, as they makes up the majority of additional ways of showing interest.