Read all the papers through once without marking at all to get a sense of how everyone did on the assignment.
Slap sticky notes on each paper after you read it for the first time indicating your gut feelings on the grade and why you'd give it that grade (B--good focus, unity, coherence; some problems in development, but not major. D--serious problems in focus, unity, coherence, etc.)
Plan your schedule so you read and final grade no more than 5-6 in a row.
Read each essay through another time without marking--have a sheet handy with Strengths/Weaknesses columns.
Write end notes based on strengths/weaknesses, then go back and indicate in margins parts that are strong and parts that are weak.
Grade a handful, take one or two graded essays and bury them in the huge pile of not-yet-graded as a delightful surprise/reward for later.