I found that the best way to get a high happiness rating and maintain it is to stay on top of them as they arrive and assign them to a task and bed right away.
If you forget to turn off your radio beacon and suddenly realize you have 12 settlers just turn off the beacon, send them to an empty settlement, then send them back one at a time to the place you want to build up.
I have three settlements with a constant 80 happiness. All of them have 7 settlers.
Each settler must have his own sleeping area. I've read that it has to have a roof and floor with a real bed, not a sleeping bag on the ground. I made a hotel at all three settlements and sectioned rooms off with walls and a short half wooden fence piece to block the view of the bed but still leave a path to walk in. I put a rug, picture, dresser, two plants, a light, radio, mounted critter, and a full size bed in each one and assigned a settler to a bed as soon as they arrive. I put their original clothes and weapon in the dresser and usually give them a minigun and an corresponding to the task I assign them to. Farmhand, guard, merchant. I made the hotels and rooms before the settlers arrived so I was ready, then I turned on the recruitment beacon.
I create a crop and assign the new settler to it immediately. I space them pretty far apart as I add more sections and put a woven wire fence piece between them. Each settler can only be assigned to 6 food, so that's 12 plants (.5) or 6 mutfruit (1). When you plant a crop of 12 of the same plant or 6 mutfruit, then assign a settler to one of the plants/mutfruit, they will automatically be assigned to the rest. You can check to see what a settler is assigned to by putting your crosshair on them while they are standing near the crop while in workshop mode. The settler and the crop they are assigned to will highlight in green.
If you have a bunch of settlers there already and try to do all this, it becomes very hard because they will take it upon themselves to start working on the crops as soon as you plant them and take over beds on their own. It seems that when I let them do things on their own the happiness rating goes all over the place.
I always have defense at least 5 times higher than the number of settlers with a combination of turrets (gun, laser, rocket) and guard posts with assigned settler.
There are some bugs. TV's and Jukeboxes lower happiness and should be removed. This may be fixed by a future patch, I don't know.
I seem to remember reading somewhere to only use 'save' files and not to overwrite them but rather create new ones each time. Then go in and delete the old ones because there is a limit as to how many save files you can have.
I've been able to maintain 80 happiness by doing this but I haven't been able to get over that.
hth.