Find where you introduce new complicated ideas. It is at an idea's introduction that the reader is most likely to disengage if they don't understand. It is no good if there is an excellent explanatory simile on the third page if the reader's given up on the first.
Tailor the simile to the idea. An effective simile is one which has verbs applicable to the thesis. For example, in the introduction, working similes into writing is compared to using a key. Because of this, the writer is able to use words associated with keys, such as "fit" and "open," which carry strong understood concepts to the reader.
The simile fits more naturally because of the familiar language, and the reader receives a better understanding of the thesis.
Show it to a friend, preferably someone unfamiliar with the concepts. In essays, similes are primarily for the benefit of explaining to a reader. If it doesn't do the job, a simile needs to be replaced in a redraft. It is often the case that the first simile employed is the first to be replaced.