You need a decent base tan first. Then it is all about using all the different products out there on the market. Even so, remember the pictures you see are ones from contests. That is like a couple of days out of the year, look at some BB'ers off season and they will be completely different.
Personally, i hit the beach in the summer, when i can (this summer its only been like 1 every two weeks unfortunately) and in the winter i'll try to hit the tanning salon as often as possible.