I really struggled with this too. What I did was place most shops (putting them next to fairly busy paths), raise their prices to the point where they had a steady stream of guests, and then sold the ones that didn't make a profit. I then trained all the staff to full and waited until I got a good month.
It might also be worth noting that I had no entry fee, two marketing campaigns going, and a large park making quite a lot of profit from rides.
There's probably an easier way to do it but I thought I'd share my method in case it helps.