To add a thought to Chelsea's answer (below), tricks aren't only deceptive (or persuasive); they're also short. Think about other uses of the word trick—we played a trick on him, she did a magic trick, it was just a trick of the light. A trick happens, then it's over.
The speaker in Sonnet 14 is terrified of impermanence. She thinks any love that stems from something changeable is suspect. She explains why in lines 11–12: "A creature might forget to weep, who bore / Thy comfort long, and lose thy love thereby!" She's talking about her lover pitying her for being sad, but the sentiment applies to all the reasons she lists. Her smile, her looks, her manner of speech, her mood—all these things "may / Be changed, or change for thee" (lines 7–8). And if the parts of her he loves change or disappear, he has no more reason to love her.
When I hear "trick of thought," I take it to mean a witty saying or insight the speaker comes out with. She describes the trick as bringing her beloved "A sense of pleasant ease on such a day" (line 6). It's something that happens one day, and then is over.
Imagine the pair on a picnic. She says something clever, and he thinks, "Wow, I never thought about it that way," or, "That was well-phrased." It's nice, but it's transient—she's not going to be able to impress her lover with every thought she has or every word she says. He might love her in that moment, but after she's finished her "trick," her thought or saying, it's not there for him to love anymore. And that's what scares her most.