Right now my game has the grocery store which I know you saw me mention in another thread, plus a small bakery. And I have 2 other retail lots that are in place, but no one has bought them yet to set up the actual store.
The grocery store is huge, on a 40x30 lot, with 2 stories for the store plus a bar on the 2nd floor where Sims can also order drinks and food when they are shopping and a 3rd story with living quarters for the owners.
It also has a playground in back so kids can stay occupied while their parents shop, and I have the grocery vendor stall from cottage living outside as well, because the store sells ALL food ingredients except red and white wrapped meat...the game won't let you sell those two things on a retail lot sadly.
I use mods by LittleMsSams so that the vendor stall and the bar both automatically get staff assigned to them, even though normally they wouldn't on a retail lot.
The bakery is just a tiny bakery like anyone would have in their game.
One of the 2 currently inactive retail lots is a potion/herbalism shop that I'm going to sell potions from RoM and Herbalism out of and also all of the ingredients for those 2 skills.
It is a small shop, with a large garden beside it for growing the ingredients and living quarters on the 2nd floor. I'll be buying and getting it set up for business with my Sim who is a teenage spellcaster right now, once she grows up and graduates from university.
The 2nd currently inactive retail lot I just finished building right now and it is where I will sell upgraded appliances like that other thread was talking about. I set up an area for kitchen appliances plus areas for vacuum cleaners, stereos, TV's, computers, sinks, toilets, baths and showers and put a 2nd floor living area on it.
I'll be buying that with the next Sim that rolls around who has traits appropriate for someone who is into handiness, whenever that happens. I randomize traits for Sims born in game so I have no idea how long it will be before it's open for business.
I'll also probably open a flower shop at some point down the road, where a Sim can do flower arranging and grow flowers beside the shop. It's not in place anywhere in the game yet. I'm hoping the new world coming with the Wedding pack is large enough that I can find a small lot there to put it.
Edit: Just remembered you asked for tips. I don't really have any other than "have fun". Retail locations can run themselves, so there's no real secrets that I can think of for having them. But they definitely are better if you have a Sim own them, because then the employees are there even when other Sims you play shop there. If the retail location is unowned there's never any employees at it...you can still shop there, but I just find it weird shopping somewhere that has no employees.