All good tips given above....and as mentioned, make darn sure you know how to stop it on a dime. Depending on the kind of machine you use, they can easily get away from you, especially on wet, sloped turf. I have a Lawnaire 28. THis thing is a beast to handle but it pulls great cores....but this spring it got away from me(my own stuipidity) and smashed thru a couple barberries.
Something else to remember...while its best to have some moisture in the ground when you aerate, if the ground is too wet/ saturated, ....you will just end up making a muddy mess and the cores removed will not be what they need to be. In order to give the customer their moneys worth, you need to plan the aerations as much as possible so you do them with the right soil conditions. Most customers will water their lawn for you the night b4 if you ask them.....and please....charge at least $10 per 1000...and thats a low number in my neck of the woods.