Some things I've noticed that might be helpful:
*Setting your profits too low won't increase your popularity by that much. Customers seem to be happiest if I have a profit of 10 beans per potion, and adjust it with each change in supply cost.
*Spend money on advertising. Spend lots of money on advertising. Even if 80% of the town thinks you're the best shop in town, if you're not spending at least $150 on ads, your popularity will be at a standstill.
*The only goon that seems to have any reliable effect at the latter end of the last level is the giant baby. It will actually keep the customers out of the other store.
*The goal is "drive competitors out of business", but really, once you get rid of the first two, the goal should be "get a net worth greater than Sneersworth". Making a tiny profit dragged things on forever. Once I set out to make a profit and increase my net worth, I noticed that Sneersworth's net doesn't actually stop increasing (or declining very, very slowly) until your net passes his. Then his net starts dropping like a rock. Even if he's only getting two customers a round, I've had no success until my net passed his.
*Upgrade fully. You're not going anywhere without the top potion maker and entertainment, and you can't keep up with potion demands if you don't have the best warehouse.
*Don't run out of potions! I've found having enough on hand to make 70 of each works well for me - and I've still run out on occasion. Just keep the supplies on hand for that much, and "refill to yesterday's totals" each round. Don't forget to adjust your price after you do so! And don't rely on the Mole - it's a waste of time.
Good luck! Hope this helps!
Edited on 10/11/2008 at 3:39:41 AM PST