Revisit the assignment and be certain your thesis is appropriate. Make sure your points support your thesis.
Reread your sources and make sure the passages quoted correspond with the points you are trying to make. Confirm that your citations are quoted and sourced correctly.
Run the spell check function of your word processing program. Be aware, however, that it will not flag misspellings that are analogous to correctly spelled words, and supplement the spell check with a thorough rereading of your paper.
Check your essay for subject-verb agreement and for language and style appropriate to the assignment. Condense or separate run-on sentences and correct any fragments.
Give your paper to a second reader for feedback on how clear your arguments are, or where you may need support.