Here's the good news: there's really no
difference between a regular SIM and a
micro-SIM. They both use the same, exact
chip, but the micro-SIM simply has less
plastic encasing it.
Naturally, it didn't take long for the
Internet to figure out that a regular SIM
could be trimmed down to the size of a
micro-SIM at little cost.
If you need to trim your SIM card to a
micro-SIM size, here are your options:
1. Head to Amazon.com and search for
a micro-SIM cutter. It
works like a hole punch. When it arrives,
you'll simply place your SIM card in the
tray and punch out a smaller size. As a
bonus, you'll also receive two adapters,
which will allow you to put the micro-SIM
card back into a regular-size SIM slot later
on.
2. Do it yourself. If you need to trim your
micro-SIM card immediately, print out
the free template below and follow the
instructions on the page. It's crucial that
you take your time with this project,
because if you make a mistake, you'll
have to purchase a new SIM card. For the
best results, use a sharp pair of scissors
and always cut less first and trim (or file)
later, if need be.

This is very informative