Also, if your foot remains on the gas and your RPM's drop, the problem is NOT your clutch. If your clutch was bad, then your RPM's would unnaturally increase for almost no reason, and your speed would remain constant, decrease, or barely increase.
Test your clutch...
Put your car on level ground with the parking brake up. Put the car into 1st or 2nd gear and rev up to around 3-4k RPM's. Dump the clutch and see if it grabs or not. Also, you can start the testing at lower or higher RPM's. Just make sure that your clutch is grabbing at mid-high range RPM's.
Another less violent way to test would be to take your car out on the highway or open road, and gun it and take it all the way to redline. Watch your RPM's on the shift.
What do they do?
If they grab immediately or only fall like 100 RPM's, then your clutch is fine.
If they fall like 500 RPM's, and you notice that your RPM's slowly begin to climb again, or don't even climb at all, then it's your clutch.
One night, I gunned my car, and went to shift at redline, and once I shifted, NO power went to my drivetrain, and my RPM's shot all the way up. Had to pull the car over and let the plate cool down so that it would grab again, and shifting above 4k was hit or miss. That means your clutch is on its last leg.
But once again, if your RPM's fall with your foot still on the gas, then it's not your clutch (thank god).