The condoms are the best solution in short time, or have a wank like 2hrs before sex. You can also focus in other things, like doing maths or something in your head during sex if you think you are going to come.
And make sure you are relaxed. If you are scared that someone is going to walk in or if you are doing it in public, you'll probably want to have it over quick. At least that is my problem with doing it in public.
In long time I would make sure you work out, if you are fit and healty you'll last longer.