Buying rentals is easy, even maintaining them at first is easy. But once you start getting to large amounts it gets a little harder.
Once you get to $20 in your account you can then purchase 100 rentals. Once you have gotten up to 300 you will need to stop. You need to take the time to look over each rental and make sure they are actively clicking. Remember the are making money for you by clicking so make sure this is done. Once you see that everyone is clicking you will keep them. If you see that someone has 0 clicks or a low amount of clicks for a couple of days click on the box next to the rented referral and then go to the bottom and recycle them. You will now get a new referral in hopes that they will click better than the last rental you had.
When your rental time is almost up your referral will turn red. So if you have 10, 3, 0 days left it will let you know. You will then have to decide if you want to keep that referral or just loose them. If the referral is not clicking you will need to let them drop you do not want to pay money to extend them if they are not clicking. If they are an active clicker you can extend them for 15, 30, 90, 150, or 200+ days. The more day you extend the more money it will be to extend them. Do this according to how much more you have in your account. Try to extend them as far out as your money will allow you to.
Doing this will build your account and bring you much more money.