1. Don't Lie
While this first principle might appear obvious, the challenge is the temptation to bend the truth a little bit for the purposes of expediency because you believe it unlikely any Canadian government bureaucrat will ever find out. Odds are they probably won't find out about your "fib" but the huge reason to avoid the lie is that if they do find out - as unlikely as that might be - you will have a HUGE problem. For starters, you'll have committed a federal offence in Canada. The lie could be considered by the Government of Canada to be a much more serious misrepresentation leading to inadmissibility than the thing you were attempting to hide.
Even an "innocent" misrepresentation can be considered by Canada to be a lie barring you from entry. Meaning that you might need to interpret the questions in as broad a manner as possible, even though that seems to be against your interests, in order to avoid any risk that Canada might take a different interpretation leading to accusations of lying that you never intended to do.
2. Very Carefully Consider Each Question Before Answering It
Although not lying may be the most important rule, and erring on the side of broad question interpretation might be the best way to avoid breaching the first rule, overly broad interpretation of the questions may in turn lead to unnecessary disclosures that could get you in trouble by telling the Government of Canada things they don't need to or really want to know about.
As I tell each of my witnesses in preparing them to testify before the IRB (or any other tribunal or court): listen to the question, and don't be helpful. Meaning, directly answer what is asked, but don't volunteer information. Unfortunately this is more art than science.
3. All the eTA Questions are Semantic Minefields
The questions sometimes have no obviously right or wrong answers. For instance, under Canadian law there is a big difference between being "arrested" and being "detained". The eTA form only asks about "arrests", and only links those arrests to "criminal offences." So you don't need to disclose detentions that didn't lead to arrest, and don't need to disclose arrests for non-criminal offences. But the legal standards you'll be judged by is how "arrest" and "criminal" is defined under Canadian law, not under the law of your country to residence or citizenship.
4. Apply for an ETA far in advance of travel
Preferably apply prior to purchasing an airline ticket. ETAs are good for five years, sort of like a passport. And if you do need to answer "yes" to one of the ETA application questions, it may take at least days and maybe weeks to sort out your admissibility to Canada, potentially leading to significant travel delays.
5. If in doubt, get legal advice from a Canadian lawyer
Formal legal advice is not overkill if it means the difference between being admitted to Canada for tourist, work or families reasons, and being barred from Canada subject to conducting months and maybe even years of paperwork trying to overcome refused entry.