Are you sure that your work is as good as it can be in every aspect?
Have you checked and re-checked the accuracy of every fact and reference used?
Are you positive that you are submitting it to the appropriate market?
Have you contacted your target market to check that they take unsolicited manuscripts?
Have you checked that the article you are aiming at is not written by commissioned writers?
Have you written to suit the tone, style, language and word length of the chosen market?
Is your work grammatically correct, with the right use of punctuation?
Is your presentation perfect? Is it clean, neat, typed on A4 white paper? Have you double spaced the manuscript on one side of the paper with wide margin to allow them to make notes?
Have you read, read-read, put it away and gone back to it and read it again?
Have you written a covering letter? This should be single spaced, brief and to the point and addressed to the appropriate editor.
Have you remembered to enclose a large stamped addressed envelope with sufficient postage for return of your work? This is not being defeatiest, it is plain curtesty and will help you see your work again, instead of it lying around with hundreds of others submitted.
I personally send my submissions by recorded delivery so it can be tracked. Keep a record of work sent, as time soon passes as work returned does not want to be sent to the same people again by mistake.