Successful study groups generally do the following:

• Keep it small--two to four people maximum 
• Set regular, consistent meeting times
• Have a set location if possible
• Discuss expectations and goals at the start of the semester (change your group early if it is not a good fit)
• Use an agenda (it doesn't have to be formal)
• Hold each other accountable 
• Encourage each other
• Keep socializing to a minimum during scheduled study meetings
• Reach out to the TA or professor for help when needed
• Be kind and respectful