I am a dictionary editor and what we do is this. We firstly employ freelance readers who read a selection of newspapers, magazines, journals and books every week and enter all the new words they come across in a database. We also keep an eye ourselves on any new words we find (either from reading them or hearing them on TV/radio or in conversations). Every couple of weeks we get together and see what new words have been spotted in different sources; we then discuss if the word in question is likely to have staying power or if (as is often the case with slang terms) it's going to be used a lot for a couple of weeks then disappear. If a word fulfills the two criteria of having appeared in at least five different sources over a reasonable time period, plus seems to be a word which will have good longevity, then it will go into the next edition of the dictionary.