You make a best, last or final offer in two situations: when you either truly can't do any better, or when you think you can get the other side to close a deal on the spot (or pretty close to it). Either way, take a light touch and choose your words carefully. Not only will the phrase "take it or leave it" miff pretty much anyone--it leaves you no graceful way out. After all, even if you really believe you have nothing left to give, a skilled opponent may show you that you do.
Better to hedge. Hedge on what you're final about, hedge on when you're final, even hedge on how final you are. "This is the best I can do under the circumstances." "Why don't you think it over for a week?" "If I don't hear from you by the 1st, I'll have to start looking for someone else." Or, if you must, go ahead and be tough, but be ready for hardball.