The most common leak is faulty "o" rings where the lines connect to the compressor.  I would check these first.  Be sure to use AC quality "o" rings and not the ordinary rubber variety.

There are also several other "o" rings where connections are made.  The dryer and the needle valve are two I can think of.  You might also have a leak in a rubber line, but I would check the fittings first.

Another source of a leak is the seal behind the compressor pulley.

George