When using Twitter, you shouldn't go crazy following others in the hope that they will follow you back. But even if you're only following, say, five people a day on average, then you'll end up hitting the two thousand limit in around a year. If you don't have at least the same number of people following you back, then you can't follow any more. Then you have to start unfollowing people from time to time if you wish to follow those who are already following you.
There are several tools you can use to do this. I've been using both Friend or Follow and JustUnFollow. I think the latter is better for choosing who to unfollow simply because of the way it's laid out. (With the former all the tweeps you're following are shown in a grid and you have to mouse over their photos to see their details, after which you can decide what to do. But with JustUnfollow all that information is laid out in front of you already and you just place ticks in boxes.)
If you are going to use one or both of these sites (or others like them) then you should be careful not to overdo it. Regularly unfollowing lots of people can result in account suspension.