Choosing which version of an article to cite I sometimes find an article published in several conferences.


  If similar material is published in different conference proceedings,
  how do I choose which one to cite?
It's a little unusual to have the same or even similar material published in different venues. First of all, you didn't say whether your area uses conferences as primary publication venues or not - I'm assuming it does, otherwise why even cite a conference publication. 

In that case, what exactly are you citing in the article ? If it's a specific result (empirical, theoretical etc), then probably the oldest venue where it appears is the first occurrence of that result, and should be cited. If you're citing background material or motivation, then either the first paper that discusses the relevant motivation, or maybe a survey article ? 

At any rate, the key is to understand exactly what you're citing the article for, and find the oldest occurrence of that concept.