First step is to get the right background material and light it properly. (evenly lit over full background, actors far enough in front of material to minimize blue spill, no shadows from actors on screen, avoid costumes same color as screen)
If you've done the above correctly you can pull a good matte with even basic keyers.
For better keying look into Primatte or Ultimatte plugins.
Note that for video (especially DV) greenscreen is preferred since it has more resolution given the 4:1:1 sampling of DV.
