What do you usually do once your sim completes their aspiration?

I pick aspirations that fit the character. So if I have completed 3 or so and don't find one more that fits the Sim, I don't pick another.

I love to buy traits from reward store, means I keep selecting what ever aspiration my sim currently is going to get some points from :) After my "essential" (gladly both are pretty cheap)reward stuff, is bought , I try ti stick at one aspiration.. (that last about a sim day, then I notice I am jumping between different aspirations again.. ) :)

I have my own goals and ideas for what I want my Sims to accomplish. I pick the aspiration that works best for what I have in mind for them, and then usually don't pick another aspiration once they're finished.

Pick a new one.

I pick one that goes with what stage they are at in their life. Such as, once they become parents and have completed an aspiration that pertains to money or a career, I give them a family oriented aspiration. But as soon as they have all the new traits I think they need, I don't give them a new aspiration.

You can switch aspirations at any time. You don't lose any progress on the other. You can always go back when the situation shifts. If you achieve one, like master painter, you could change it to romantic soulmate. Then something else. One aspiration is not 'sledge-hammered' on your Sim. It's actually one of the most flexible things in-game.
You can achieve parts on many aspirations. Without completing them.

I have a question don't know if you could help me out. I just became a world famous celebrity and have to pick a new aspiration. If I choose another aspiration am I not a celebrity anymore?

I also play long life. Once the Sim ages to teen, I like to do the fitness and painting aspirations. They are both useful later in the Sim's life. After that, I choose aspirations based on what I want the Sim to do. With the University pack added, I'll use that aspiration while the Sim is in college.

I almost always pick a new one. I had one child who was going to be aging up soon, and who had no interest in any of the other child aspirations, so I held off and let that child go without an aspiration for a little while. Otherwise, I have always picked a new one that fit their personality. I really wish there were more aspirations to choose from.

I've always just picked another one unless there are no aspirations left that fit my sim's personality or desires. Then I either dont bother picking a new one or pick one that I know they will never complete.

Sometimes their stories go in a direction I didn't expect and when that happens I can always pick one if it fits.

In fact, since I'm a wimp and play with ageing off :D I usually only have between 1 and 3 completed no mater how long I've had the sim.

One of these days I'll try a game with ageing on, but it wont be on my current save.