Is there a way other than add more datafile to tablespace for space ran out issue?

Hi:

Usually when space are used up in the tablespace, I always add new datafile or extend the current datafile to a larger size. Somehow one of our very senior dba has another way to tackle this issue. He usually adds more space only at the very last resort. He told me he will reconcile the current space in the tablespace first. I don't totally understand how he does reconcile and he would not tell me. But often times, it does resolve the issue without having to add more space. Do any gurus here know what method it could be? Thanks