"Mechanical" tuning fixes

Although proper initial tuning and the use of alternate fingers are the things you should look into first, some adjustments to the instrument itself may be called for when there's no feasible way to correct tuning otherwise. Many of these you can do yourself. 

This article is a summary of possibilities; the video below has more details.

Overall tuning

If your wood instrument is consistenly way out of tune through most of the registers, it's possible, though not probable, that the main bore of the instrument may have changed. This is a rare thing, but it happens.  (The other possibility is that your instrument was crap from the beginning. I hope not!)

You can't fix bore problems yourself (only a few techs. are expert enough to do it, and it's extremely expensive), but one of the things you can try is to tune the open G with the barrel, then tune the lower C by pulling out or pushing in the lower joint. Compromise between the two if necessary. (In rare situations, you might want to tune the low E by pulling out the bell, but isn't often required, and will affect the middle-line B much more than the low E.)

It's possible that using a shorter or longer barrel might bring you closer, in most registers, to a tolerable degree of out-of-tuneness. As I've noted in other places, though, changing the barrel length will have a much greater affect on the notes in the throat register and the upper clarion than it will for notes lower down on the tube. If your instrument is so wildly out of tune that you need try a barrel more than 2mm longer or shorter than what you have now, you probably will not be successful. 

It's also possible that your mouthpiece may be one of the "high pitch" mouthpieces that sneaked into your case at some point. These mouthpieces, though rare, are out there and are preferred by many jazz players because they compensate for the "loose" embouchure that many of them use.

Important! Some people try to get their techs to shorten a barrel or mouthpiece in a lathe rather than replacing them.  This is a mistake because the bores of these components are almost never pure cylinders (they're tapered or otherwise modified) so doing so may not only alter your sound, it might throw your overall tuning even more out of wack!

Tuning individual notes

Tuning of individual notes is affected mostly by three things:

• Pad height

• Tone hole placement

• Tone hole dimensions and undercutting (only done by experts)

Tuning individual notes: Pad height

Pad height is the easiest to deal with.

A pad that is too close to the tone hole will flatten the pitch and often make the sound stuffy. (Sometimes this is due to the wrong pads being installed on the instrument manufacturers of cheap instruments (CSI's: "Clarinet-shaped Instruments) or by inexpert technicians.) 

This can be corrected in three ways: 

• Gently bend the key cup up. 

• Decrease the thickness of the cork key bumper by sanding it down with an emory board, little by little, increasing the distance between the tone hole and the pad. 

• If the key has an adjustment screw,  adjust it that so the pad opens more.

In any of these cases,  especially bending keys, it's important that you make gradual adjustments, testing after each operation.

A pad that is too far away from the tone hole may may cause the pitch to be sharp.

This can be corrected in three ways: 

• Gently bend the key cup down. 

• Increase the thickness of the cork key bumper by adding thin strips of cork glued or durable paper with contact cement, then sanding down if necessary..)

• If the key has an adjustment screw,  adjust it that so the pad closes more.

Keep in mind, though, that while lowering the pad will correct tuning, it may also make the sound of that note stuffy.

Tuning individual notes: Altering tone holes

It's nearly impossible for a non-expert to make a pitch higher by altering tone holes. Don't try it! However, in many cases it's possible for the rest of us to lower the pitch of a tone hole note (mostly in the upper joint) by building up the top edge (toward the mouthpiece) of a tone hole without too much difficulty. (Building up the bottom edge of a tone hole does nothing other than to make the sound stuff; it does not alter tuning.)

This video covers, in more detail, all the techniques for altering tuning that I've described above, but working with tone holes starts at 21:22:

I've used tape in the past, but if I ever need to do this again, I'll use the material Tom suggests in the video; it's easier and more accurate. Tone hole tuning with tape is a major pain.

(Whatever approach you use, it's critical that the material you use does not extend into the bore of the clarinet.

Finally: The old lament

Look to yourself first.  If you're playing with a slack embouchure or a pinched embouchure, your overall tuning may be flat or sharp accordingly, and playing with both insufficient breath support and a slack embouchure will make you significantly flat.  As always, the clarinet may not be the problem!

Over and out!