Personally this is the way I think about this:
1) If I see something on offer and have to think about whether I can afford it or not then the answer is easy - I can't afford it
Simple answer - don't buy it!
2) When you do not have enough money but want to buy the item using an overdraft, taking out a loan etc.
Simple answer - don't buy it!
Perhaps I hold traditional old fashioned ideas but not having debt is a very good feeling. You have to ask the question Do I need it or Do I want it: there is a big difference. Then when you have the funds available go out and buy it without giving banks and loan companies ridiculous rates of interest.