What considerations do I need to make when mounting heavy objects on plaster walls?
Your task is to find the studs..  

Electronic versions of stud finders abound, plaster is difficult for most of them. I have a 1/2 dozen of them..  I keep hoping.  The problem with plaster is the lath used to support it. The lath can be a variety of materials: wood strips, metal mesh and even pasterboard.  All of these materials are attached to the studs and THEN the plaster is coated over the lath. Two newer scanners aimed at professional use Milwalkee scanner    Bosch scanner  .  They both will detect electrical wires and some pipe, but you are on your own for proper interpretation.

One technique is to verify stud location is via drilling a short depth (not more than an inch to verify a stud) 

Drilling in plaster: masonry bits or ceramic tile bits are the best to use to cut through the plaster with fewer cracks.  Driving nails is a plea for cracks.  The drill diameter should match the screw diameter, to not place stress on the plaster.  The actual screw holding will be done by the stud.

Once a stud is located, wood screws should be imbedded 1/2" or more, depending on the weight.  The earlier comment regarding a "buffer" board spanning the area is a good one, especially if the place you want to attach is between studs.

Finally, patching small cracks is not hard, use Durabond setting joint compound to mimic plaster.