Well, there would of course also be the possibility to use the testing cheats - you could then, holding down shift while doing so, click and drag the relationship bars in the desired direction. I used this once in a household of 4 roommates which all had different careers, but which I wanted to be close friends. You could also build up the relationship in a more natural way, but then use the cheat to prevent it from falling again.
Other things I found to be useful (and that were not yet mentioned by others in this thread), are:
- precook a meal when you have time, then put the leftovers in the fridge and when the first sim is hungry, gather the whole household together and let them all eat. It doesn't matter if not everyone is hungry, just don't let them eat up, the important thing is, once you do that, they're all going to get hungry again at about the same time.
- I make sure all beds in the house give the same amount of energy, so everyone needs the same hours of sleeping. I usually let one sim who doesn't need to go anywhere (work, school, etc.) wake up a little earlier and prepare food, and use the bathroom, so they don't block it when the others wake up
- I also try to give my sims who have to go to school/work kind of a natural daily rhythm, i.e. I don't let them sleep as soon as they come home, because then they get up in the middle of the night and get tired when they have to go to their duties. I try to put everyone to bed at the same time, and let the ones that don't need that much energy simply sleep a little longer. This way they don't miss each other's wake time.