An article written in the Globe and Mail by employment lawyer Daniel Lublin covers just about everything you need to know about severance.  Below are a few myths the article dispels that might surprise you.
Myth #1: You are entitled to NO MORE than the statutory minimum
Unless you are a transient worker, or have signed a pre-negotiated severance amount in your employment contract, severance is negotiable, and you should almost never accept the first offer.
Myth #2: Severance is a straightforward calculation based upon length of employment
Rather, it is calculated based upon the length of time it would reasonably take an employee to find suitable work.  Recently, the Ontario Superior Court awarded an employee 26 weeks compared to the 9 he was initially offered because of a grim likelihood of reemployment.
Myth # 3: Severance is yours to keep, even if you find a new job
Clauses are often present in severance agreements to prohibit what Lublin calls “double-dipping” – getting a paycheck while also receiving severance from your former employer.
Myth # 4: Poor performance effects severance
Unless you have been dismissed for cause, performance has nothing to do with severance and “both stars and incompetents should receive the same payouts if fired.”
Myth # 5:  Employees are owed a “good reason” for dismissal
This is false.  Employers are permitted to fire you just because they don’t like you, permitted (1) Their reason isn’t based upon a prohibited ground of discrimination, and (2) Proper notice or pay in lieu has been provided.
The full article is well worth a read, as it contains a more exhaustive list of often misunderstood facts about severance.  It is important that you are financially comfortable while searching for new work.  Knowing your rights will ensure that you get what you deserve.