Make sure parents in the classroom are aware of the general rules that the students must abide by. For example, if you have specific rules for bathroom use (students must go in teams, only one trip allowed per session, etc.), fill the parent in so the class is consistent. Also, let parents know how you handle discipline issues and behavior in the classroom, so they can help, not hinder, what you already have in place.
Let parents know the limits to what they can and cannot do in the classroom. If you do not want parents discussing personal issues with students, let them know. Also, make them aware of student privacy issues when it comes to the student's family life, grades, etc. Communicate with parents and explain to them any areas that are out-of-bounds. For example, if you do not want them in the desks or cubbyholes, communicate your desires before they volunteer.
Have a schedule for the days parents volunteer and be specific on the help you would like to receive. If you want parents to administer a spelling quiz, read a chapter of a book to a group of students or fill out book orders, let them know at the beginning of class. The more specific you are on the help that you need and want, the better the classroom will flow. Consider creating note cards to hand to parents with specific tasks as soon as they arrive.
Consider having a parent-volunteer orientation a couple of times a year. This is a time to share your expectations and excitement about parental involvement. You can show parents around the classroom, where you keep supplies, and how certain things work. You can also explain your overall philosophy on a successfully managed school day and how volunteering will work.