I was told to get a police certificate which I have done, £10. The only other thing that I haven't read here that might be useful is that it is best advised not to have a one way ticket into Botswana as this comes with a lot of questions. Maybe they think you are a journalist looking to get into Zimbabwe or something?
Where did you hear this? What about buying a one way ticket to Johannesburg, South Africa? I don't think I could afford to buy a round trip ticket