It is generally best to assume that your essay will be at the bottom of the pile and will be read by a weary, grumpy supervisor in the early hours of the morning. You should therefore make your work as easy to read as possible. Use a boring 11 or 12 point font (Arial, Times, etc.), 1.5 line spacing and include decent margins (say 2.5 cm). Allow white space between your paragraphs. Finally, always proof read your written work before submitting it to eliminate simple errors.
It is important to minimise spelling and grammar errors.