How would I go about (in these negotiating times) to avoid such a scenario?
You can't, although a well-thought contract reduces the chances of loss. Even if you hire an expensive attorney and sue the buyer or company, you still are at risk of losing everything because there are many incompetent lawyers and many corrupt judges. The judge presiding your case might be literally a felon whose manifesto in court is to favor "anybody who's powerful".
Although you refer to negotiations times, it is important to keep in mind that negotiations are just a process in preparation for something enforceable and consequently much more important: the contract. In the event of discrepancy, a contract will almost always prevail over any representations made during negotiations.
That being said, the first thing for you to determine is what exactly you would prefer to accomplish (i.e., to sell your shares or to keep them) and why. This is not clear from your post and comments in other answers.
For instance, your mention that
I can only pocket it through salary agreements
suggests that, instead of receiving a lump sum, you would be paid in installments under color of an employment relation.
But then you mention some concern about dilution of your shares, albeit without elaborating what relation you think it bears with a breach of fiduciary duty or with the event of selling all your shares at once. What does dilution have to do with the salary agreements you mention?
You don't specify whether you currently perform any functions at the company, but your mention of salary agreements prompts the question: Is an employment relation necessary for the sale? more specifically, is it necessary to intertwine a shares transaction with an employment relation?
Your status as employee & former shareholder could give the counterparty an opportunity to disrupt your entitlement by tortiously alleging that you have a conflict of interest.
Will your employment be at-will? how will your "salary" agreement differentiate between compensation insofar as an employee versus the proceeds from the transfer of shares?
Is it better for you to (1) sell all your shares at once and be paid in installments, or (2) sell them in batches at different times so you retain the ability to decline further transfers/agreements if the whole matter is not working for you?
These are only few of several issues you need to decide before you get to the point of reflecting your conditions in a legal document (i.e., contract) for the sell-off of shares.
It also appears that you are confusing the meaning or applicability of fiduciary duty in your case. The duty arises in contexts where a party expectedly places special reliance on representations made by his agent (such as an adviser, an attorney, etc.) or person of superior position. Contract law certainly is premised on a covenant of good faith and fair dealing, but fiduciary duty seems inapplicable in your matter because it is about an arm's length transaction: a context in which two shareholders (namely, an "angel" prone to getting ticked off and you) have opposite interests.