1) To play it completely safe, commit to either paragon or renegade. I'd go paragon of you don't want to make yourself sad. There are glitches to exploit so you don't have to do that, but those would be spoilers.

2) Also prioritise putting points into the skills that open up paragon/renegade speech options. Should be easy if you followed point 1.

You could possibly consider the following spoilers.

3) If anyone in game 2 has an argument, don't take a side. Neutral dialogue options should be available if you followed points 1 and 2.

4) Get every ship upgrade you can in ME2.

5) At the end of ME2 don't give anyone with dual specialities a job. Pure biotic or pure tech only as applicable. Unless someone is needed to take a leadership position, then choose someone you know has been in charge of team/s before. If you need to send anyone back to the ship send someone who prefers a lab to combat anyway.

6) The above should give the optimum start to ME3

7) Assuming you are playing legendary edition, get at least 7800 total military strength. This should be a bit easier if you followed the above.