The basics, at least in my area,  in answer to the question uptopic:

Prematures may start out recieving only IV fluids.  Depending on many
    factors, these fluids may contain just sugars/salts or
    proteins/fats/micronutrients.
They then advance to IVs plus some gavage feedings (EBM, abm, or
         fortified EBM also somtimes diluted EBM).
    The gavage feedings may go to the stomach or to the small bowel.
    Some very early, extremely small, priming feedings may be given
         before the baby is really ready to tolerate much in his/her
         gut.
    The gavage feedings may be continous or by bolus every 1 - 3 hours.
    The tube may be in the mouth or a nare.
    The tube may stay in for set periods of time or come out after a
         feeding.
The IV fluids are decreased as the gavage feedings increase.
Some prematures skip the gavage phase and/or the IV phase if they are
     mature enough to go to direct by mouth feedings.
The IV is removed when "full feeds" are reached, unless it is needed
    for reasons other than nutrition.
Once a baby can tolerate "full feeds" if s/he has been gavaged attempts
     will be made to start oral feeding.  This is where the
     breast/bottle/cup/other alternative methods hoopla starts.
Gradually, more and more gavage feeds are given over to oral feeds
    until the baby no longer requires any gavaging.  Up to recently
    this was the point many went home in the US.  Now, I hear rumors
    some are being sent home with some gavages still needed.  I wonder
    if the push for quick discharges is preventing the use of the
    breast before bottle approach?
Questions about the prematures ability to demand feed and do well
    exist.

Paula Meier has done considerable research on BFing prematures.  Her
articles (about 30) from 10 separate studies are well worth the effort
it may take to obtain them.  She is not always the lead author but you
can see her touch in all the articles.  They are in a wide variety of
journals.  She has sent a bibliography on request via snail mail.  She
also provides a bib at any session I've heard her speak at.  Try to
hear her if you get a chance, well worth it.

Carla D'Anna