Don't rely on your memory for the dates of when things happened. Find the paperwork that went with the event or look it up online or in your calendar, etc. you don't want to be accused of lying simply because you weren't careful.
Do pull your credit report to double check what is on it before completing the financial questions. Save the report! It will be good proof later that you answered those questions honestly even if you omit something.
Do make a file and put into it copies of any documents that you look at for information while you're filling out the questionnaire. You won't remember in the interview what you were relying on if the information get challenged.
Don't wait until the last minute to fill out the questionnaire. It will take longer than you think. You don't want to be accused of lying simply because you rushed and made mistakes.
Do document (with email or a written memo to your file) any advice the security officer gives you about how to fill out the questionnaire or whether things need to be disclosed. The officer won't remember doing that later when you try to get him or her to corroborate the advice to the investigator or adjudicator.
Don't err on the side of leaving things out! Err on the side of disclosing them. If you're really not sure what to do, talk to your security officer or a security clearance lawyer.
Do print out a copy of your completed, SF86 and put it in a safe, protected place where you will remember to look for it later. (Perhaps with your tax returns?) You'll be glad you did when it comes time to update your clearance.