You've already received tons of advice so I hate to pile more on but I can't help myself.
Being in my line of work I pay close attention to people that have come and gone as vendors on this and other forums since that's my customer base. One thing I see happen frequently is people taking on more work than they can handle. Customers start getting upset because you're running behind then the vendor gets frustrated and it all spirals downhill. Since your work is pretty much reliant on the artistry and effort you've put into it, it's going to be all on your shoulders. So make sure you only take on as much work as you can comfortably accomplish. Be upfront with people about how long it's going to take to get the job done.
The other thing I see a lot with sole proprietor businesses, is they tend to short change themselves on wages. Meaning they don't pay themselves what they are worth. Ultimately if you don't value yourself highly enough, others will also start to undervalue your effort. Charge your customers for your time because it has value and you deserve to be compensated. Feeling like you aren't making enough to be worth the trouble is the first thing that will burn you out.