look at past questions.
Some students make their essays narrative-heavy, which can drag their marks down, i.e. retelling parts of the plays and linking the events rather than the themes. Practicing essay plans and writing essays in timed conditions now should help.
Also, it sounds obvious but make sure that you know the plays really well! The 2015 Greek tragedy exam had a context question on Medea taken from 386-391 which starts with "Well then, suppose them dead. Which city will take me in?". Most students took this to refer to the children, but in fact it is too early in the play to refer to the children and actually refers to Jason and Glauce. This then had a bearing on the 5 mark 'what's just happened?' question and also the 10-mark "how is this in keeping with Medea's character?" question.
Likewise, one of the essays asked about the beginning of the plays and their effectiveness and it was surprising how many answers totally forgot the fact that the Nurse opens Medea...
Hope this helps!