There seems to be quite a bit of misinformation in this thread. Forgive the length of this post but I wanted to reiterate the proper method of rev matching during a downshift as it was taught to me.
Some general tips about downshifting and rev matching. This has been stated before in previous posts buried in the thread.
The entire point of downshifting to a lower gear (in day to day driving) when slowing down is to keep the car constantly in a gear and under power. Or going into a lower gear in order to exert more power from your vehicle (passing a car on the highway etc, impromptu stop light granprix's).
You want to do this for the simple fact of being able to apply throttle to avoid any emergencies that may happen. That 1 second it takes to shift from neutral into a gear can be the difference between an accident and being able to apply throttle and avoid something behind or in front of you. Of course this is only when applying the brakes isn't the most logical thing to do in an emergency situation.
Also, there is absolutely Zero
reason to double clutch a manual transmission that uses syncro's (i.e. any modern trans).
The logic behind rev matching is to sync your flywheel (by bliping the throttle) to your tranny speed, in order to downshift into a lower gear and stay under throttle through a turn or to use the engine to brake.
Simply put you want to match the flywheel RPM to what the RPM of the transmission would be in the gear you select. So for example:
You are in 4th gear going 40mph at 2500 rpm (hypothetical numbers)
Hypothetically 3rd gear at 40mph would cause the RPM's to sit at 3500rpm. So logically when you shift into 3rd gear you'll want the flywheel to be spinning at 3500rpm so that when you let go of the clutch pedal your engine speed will be spinning at the same rpm as the transmission in 3rd gear.
So it's as simple as
1, depress clutch
2, blip throttle so RPM's are a tad above targeted RPM (3500rpm in this case) while shifting into 3rd gear
3, engage clutch
4, feel like a boss
Putting the RPM's above slightly what you intend is done to compensate for the drop in RPM's while you shift/engage the clutch.
It's really as simple as that, but it's a learned technique. No one can tell you the correct RPM's for every gear at every speed. It's something that varies drastically from car to car, mods, wheel sizes etc. It can only be learned with practice.
This leads into a more advanced technique that I'm sure many have heard about called heel/toe. The purpose of the heel/toe technique is simply to be able to apply the brake while performing a proper rev matched downshift.
It just involves all the steps mentioned above but using your right foot to apply the brake and blip the throttle at the same time. This one however can be dangerous when practiced in traffic for the first time. But can make driving very fun even in ho hum every day traffic.
For fun, here's a you tube video of Walter Rohrl an amazing group B rally driver from way back in the day. You can see his pedal work using heel/toe, left foot/trail braking and some amazing speed.