Just wait for the rpms to fall. If it's taking an excessive amount of time to fall. Something else is wrong.

You blip the throttle to downshift to match rpms. This prevents you from a shock of deceleration to the drivewheels. Which would spin you out if you were slowing down for a corner while racing.

For upshifting. If you pull the rpms down when you release the clutch in the next higher gear, you are shifting too quick. Fine for racing but not for normal driving. If the rpms pull up from idle, then you are shifting too slow.

 

The RPMs take a good few seconds too fall, from 3-7 I believe (though I can't be sure that's accurate at all). That helps with the idea of shifting a lot though, thanks!

 

Even if it's closer to your short time of 3 seconds. That's too long for rpms to fall for a normal upshift. To drop from about 2500 to 1500, it should take about 1 second.

It's not a Mustang but it will give the the approximate idea of the time for rpm to fall between shift during normal driving.

http://www.youtube.com/watch?v=g8LhyPhMWC4

 

I started watching it closely and it falls a good 1000 RPMs depending on the temp I believe, or some other factor I'm not noticing. In the morning it takes a good second to fall a good 1000 RPMs on a straight road. By the afternoon it seems to take a good 3 seconds to fall the same.

And it's no where near as fast to fall as that trucks did.