Research your part in the play. Know the back story of your character and learn about personality traits you can act out to fully come into character. For example, if the person you are portraying comes from a royal background, research how royalty lives and the costumes and etiquette associated with that status.
Study your lines often. Work with other actors in the play to create a believable chemistry on stage. Say lines with people not associated with the play and ask for feedback about how you are saying your lines and if your emotion seems genuine.
Understand the stage. Acoustics on a stage help to project your voice--but only if you are facing the audience. Practice not only your lines, but also take stage direction notes on your script and practice body movements as they correspond with your lines.
Speak clearly and enunciate. On stage, you must exaggerate your words and actions (but be careful not to over exaggerate) so the entire audience sees and understands what is happening in the play. During rehearsals, have people far away from the stage to see if you're words and actions are clearly heard and seen.
Improvise lines if you can't remember during the play. Improvising a line or two in a scene can save you from totally losing your train of thought.
Hire an acting coach to teach you special methods and tricks for stage performance. A coach can guide you in how to improvise, properly take stage direction and learn lines more efficiently.