You can lower your mortgage interest rate by putting down a higher down payment, to begin with. Loans with lower LTVs come with lower interest rates.
Higher credit scores also help. Borrowers with a 780 credit score can borrower money cheaper than those with a 680 score.
Beyond that, it helps to have a strong track record as a real estate investor. If you can prove your past successes to the lender, they often offer lower rates.
Similarly, it helps if you’ve already established a relationship with the bank or lender.