A few others, not necessarily specific to apologetics, but good to remember while engaging in it.
2 Timothy 2:14:
Of these things put them in remembrance, charging them before the Lord that they strive not about words to no profit, but to the subverting of the hearers.
We should not strive over useless things.
Mat 7:6
Give not that which is holy unto the dogs, neither cast ye your pearls before swine, lest they trample them under their feet, and turn again and rend you.
Sometimes a person's obstinence cannot be helped by our efforts.
2 Timothy 3:16-17
All scripture is given by inspiration of God, and is profitable for doctrine, for reproof, for correction, for instruction in righteousness:
That the man of God may be perfect, throughly furnished unto all good works.
It's good to remember that we need full preparation and grounding in scripture.
Heb 4:12
For the word of God is quick, and powerful, and sharper than any twoedged sword, piercing even to the dividing asunder of soul and spirit, and of the joints and marrow, and is a discerner of the thoughts and intents of the heart.
Another reminder that it isn't our efforts that change hearts, but the Word of God.
And finally, I often think this passage went through Bahnsen's mind during his debate with Stein:
Job 6:20
They were confounded because they had hoped; they came thither, and were ashamed. 