This is a difficult question to answer without a detailed consultation. Typically, you negotiate a favorable severance package through leverage. You gain leverage by a myriad of methods, the threat of litigation is just one. Many industries are concerned about their reputation in the recruitment of key employee, and will often provide a favorable package to insure that you not become disgruntled and damage their ability to hire other employee. In sum, their is no one size fits all negotiation strategy. By the way it was likely the woman sleeping with the boss threatened a sexual harassment claim to attain her severance.
First, you need to understand that there is no "entitlement" to severance pay. Only if the employer wants to offer it, will you get it. It is a "gift," with strings attached. In exchange for the severance, you are agreeing, among other things, to release the employer from any and all claims you may have against it. The best way to negotiate a better severance package is to hire an experienced lawyer who can help you analyze whether or not you have any claims against the employer, and then using the threat of those claims to leverage a larger severance package. Without such leverage, you are unlikely to achieve a "better" or "larger" severance package.
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