I think there might be several options: 1) Лучше ДЕЛАТЬ это после того, как я ухожу. This is regarding situation when someone does regular action. For example, the cleaner comes to your office every day for cleaning and he/she starts do that when you are still in the office. You don't like it and say to her "Лучше делать это после того как я ухожу." 2) Лучше сделать это после того, как я УЙДУ. this is used in any other situations.

Правильно: Лучше сделать это после того, как я уйду.

That depends. 1. If you, say, discuss with a janito that cleans your office, when to take out all the garbage, you can tell 'em to do that after you're gone. Like on dayly basis. In that case you say: Лучше делать это после того как я ухожу. 2. Now, that same janitor asks you, when you'd like him to vacuum your office TODAY ONLY. During your coffee break, or after you're gone. And you tell him, it's better to do that after you're gone by saying: Лучше сделайте это после того как я уйду.