You have to be careful with this rule. If you only have cash in the account, just keep the cash below $50,000. But if you have an investment account, it's not as simple. The value of investments goes up and down throughout the year and if at any point in the year, the total value of those investments goes above $50,000, the account is no longer exempt from filing requirements. To make it more complicated, while many types of investments are included in the exemption, some aren't. This is the full description of the exemption from CRA:
(b) a trust that hold assets with a total fair market value that does not exceed $50,000 throughout the year, if the only assets held by the trust throughout the year are one or more of
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(i) money (note that money does not include collectible gold or silver coins, or gold or silver bars),
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(ii) a debt obligation described in paragraph (a) of the definition "fully exempt interest" in subsection 212(3),
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(iii) a share, debt obligation, or right listed on a designated stock exchange,
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(iv) a share of the capital stock of a mutual fund corporation,
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(v) a unit of a mutual fund trust,
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(vi) an interest in a related segregated fund (within the meaning assigned by paragraph 138.1(1)(a) of the Income Tax Act, and
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(vii) an interest, as a beneficiary under a trust, that is listed on a designated stock exchange;