The Lakers seem to be the team who do the best job of always being in contention. But even they can't stay on top consecutively. I'm of the opinion that you can't be in more than one mode at the same time. Right now, Boston and Los Angeles are in "contending" mode. They can make some adjustments to the team, but for the most part, the team core is set. A team like New York is in "rebuilding" mode, trying to find the pieces to be competitive. Atlanta, I would think, is in "development" mode. They've got good pieces that they need to develop in order to be competitive. And very rarely do the Lakers depend on their draft picks, because they usually have winning seasons and late picks.
The idea, I believe, is to trade your players while they still have market value. In other words, you want to trade them while they're still good. When I say market value, I mean not only "expiring contract" value, but also that the player still has the ability to play at a high level. Basically, once you realize that the core isn't going to get any better, you trade them. The team then goes downhill for a while as it switches to rebuilding mode. The temptation is to hang on to the stars as long as the team is doing well. However, that makes rebuilding for the future much harder. Better to go ahead and trade them while they still have value and start rebuilding before the team hits rock bottom. In short, when should you trade Ray Allen? That's the question for a good GM to answer.
There may be situations in which a team may hang on to a star, or look to acquire an aging marketable star, in order to keep the fans coming in. LA doesn't have that problem, so they've never been shy about trading away their stars to quickly get back to rebuilding mode. Maybe that's why they've been so successful at being competitive.