Two good things that will help are keeping the servers reliable (which if you are a reseller then pick one that manages their servers well), and customer service. When you start hosting people, businesses etc... you can get clients from anywhere in the world and therefore, 8pm at night for you could be business hours for them. So be prepared to get support, sales, billing enquireys at any time of the day.
If you can provide a good service then you are on to a winner!
Also keep your records up todate of customers and your billing and taxing. It only requires a little bit of time a day, but if you leave it till the end of the year then you have a lot of work to do to tidy things up.
There are other things... but I will let others chime in