These are my impressions from watching a lot of Kitchen Nightmares / Restaurant Impossible, reality shows in which celebrity chefs 'save' failing restaurants and try to teach them to succeed:
1) An operating team that knows what they're doing and works well together. If you don't know how to cook, don't open a restaurant without someone who does (really does). Learn everything you can about it, but have a clear division of responsibilities and decision making. Don't lock your chef out of making choices about the menu or how the food's prepared if you're not a chef. Have clear leaders in the front of the house and the back of the house, with respectful, professional communication between them and everyone else. Have one clear overall decision-maker.
2) Track your freaking costs and revenues. All of them! A restaurant is a business, first and foremost. Understand your food costs. Understand the cost of portion sizes. Don't skimp on a POS system -- the time and accuracy it saves you over going through paper tickets in calculating revenue / most popular items / most valuable waitstaff is massive. Track your most / least valuable days and times. Don't overstaff during slow times.
3) Keep your menu small. The more items you have:
Something-for-everyone is like pleasing-all-people; it's a recipe for failure. Understand what kind of restaurant you want to be, choose a core style of food and set of ingredients, and vary with sides, preparation, and spices.
4) 'Good customer service' means setting standards for your staff and enforcing them. People who seem polite and friendly in an interview don't always remain so in the stress of the dining room, or as autopilot sets in after months of working. Watch your staff and coach them. Train them to begin with in how to interact with customers, expectations for how often they should check in with them, particular things they should say (May I tell you about our specials? Would you like a wine recommendation with that? May I tell you about our desserts? How was your food this evening? Thank you for coming in!); these things don't come naturally, they're part of a system you need to train them in.
5) Clean. Thoroughly. Every day. Dirtiness builds, and you and your people don't notice it because it's happening gradually in a place you go to almost every day. By the time you've noticed it, it's too late to easily do anything about it. Your customers have already noticed it on their first time in the door, and left you bad reviews or simply not come back because of it. You're already attracting bugs and mice, and they're going to keep coming back even after a good cleaning.
Every item that would have taken five minutes to wipe down the night of is now going to take an hour of scrubbing with industrial grease cleaner. Create checklists, have your staff take them seriously, and be vigilant as time goes on.
Not failing is a big part of succeeding as a restaurant. After that, what gets more people coming in and coming back are two simple things: good marketing and great food.