A dirt bike clutch is there for 4 reasons:

1. To shift from neutral to 1st (or 2nd) gear and get you going from a dead stop.
2. To keep the bike from stalling under heavy braking.
3. To keep the bike from stalling while in the air, by pulling the clutch in and twisting the throttle a few times.
4. To launch the bike out of a corner.

Those 4 reason above are the only time you should really be using your clutch! You don't have to use it while shifting - up or down - it will only slow you down. Your bike will shift up and down just fine without using the clutch, although shifting up is harder then shifting down, since your engine is under heavy load when accelerating.

The hardest time to up-shift without using the clutch is probably right out of a turn, going from 2nd gear to third gear for example, since that's when you put the most load on your engine. If the gear won't shift up, you then have 2 choices,  and one of them is using that good old clutch. The other method is one I use very often, and that's lessening the amount of engine load by doing a tiny "bunny hop" and barely decelerating at the same time.
Let me explain...when you come out of a turn and you're on the gas, most of the time you're sitting down on your seat, or you're barely standing up, either way you're putting a lot of weight on the rear wheel and increasing engine load. You can do a small "bunny hop" by pushing down on your foot-peg hard with your whole body,  and releasing the pressure right away while making yourself as light as possible. This will make your bike's suspensions compress and decompress very rapidly, lessening the engine load. As soon as your suspensions decompress,  let go of the throttle for a split second shift  up at that time. If you do all that correctly and with the right timing, your bike will go into the next gear like a breeze.

This technique isn't easy by any means so don't try it during a race or something important. Make sure you practice it a lot during the week or the next time you go riding, and keep practicing it until you feel comfortable with it. You could always use the clutch in those situations (and sometime you have to, no matter what you do), but really, who wants to take the easy way out? If you want to go that way, make sure you just flick that clutch lever really quickly, just enough for the gear to slide up, and you won't loose too much time.

If you want to add anything to this quick tip, of if you disagree with something, please leave your comments below. Thanks for reading.