2. Don't post spammy crap
3. Don't post useless crap
....
tldr; Don't post crap so you wont have negative trust
This is the problem with the Trust system. The Trust system isn't to be used to grade people on their post content or quality. It's because of the thinking quoted that the Trust system has become abused and useless.
The best way to guard against negative trust is to not engage in any business transactions with anyone on this forum!
But that may be a bit tough to do if you want to be active in such transactions here. So, if you must conduct transactions be very clear about the expectations of the transaction, use escrow for payments, and vet your business partners to make sure they have a good reputation.
The Trust system is abused on this forum. People use the Trust system as a weapon to penalize people they don't like, whether they've conducted a transaction with someone or not.
Case in point, at one time I wanted to get into the escrow services business. I started offering my services for free to try to build a reputation providing that service. The existing players in the escrow business on this forum started giving me negative trust because I was trying to compete with them. I never conducted a single transaction, especially not with these people, and they gave me negative trust. It's pitiful fucking behavior on their part!