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Basic Principles
Final reed strength is a subject of debate and personal (or band) preference.
Many bands prefer or require the strongest reed a piper can handle safely.
Many (but not all) fine soloists prefer a much easier reed, one which can
be played for a long time without tiring the piper. I prefer an easier
reed still, one which I can play without tiring my lips or breathing on
any day in which I'm healthy enough to go out in public. (I'm not sickly,
just prone to the usual ailments.) Therefore I rarely forfeit a performance
engagement due to common colds, allergies or flu.
In some of the big name bands they're known to choose real lip-bleeders
and suffer for weeks breaking them in before ever putting on any drones.
Yikes.
The usual procedure is to select a strong reed but one you could blow
comfortably enough to play tunes & embellishments for some shortish
period, maybe 10-30 minutes on chanter alone, with the drones all stopped.
This is blown-in over a period of several weeks until it settles-in to
the desired strength and can be played in whatever passes for comfort in
your case, playing the most difficult music with all drones going, as long
as desired.
Strength is related most obviously to the amount of opening or "elevation"
of the lips of the reed. The greater the elevation, the more air the reed
uses, the louder and flatter it plays, and the harder it is to blow.
The blowing-in procedure is called "training" the reed. The
theory is that by leaving most or all of the cane on the reed but merely
reshaping it, you end up with a longer lasting reed which is less prone
to changing tuning rapidly while you play and during periods when you are
resting.
Pretty soon, within just a few days, the reed becomes less and less
changeable as it warms up, and generally easier to blow.
Of course all this presumes a well assembled reed made of good quality
cane.
New reeds are strong for 3 common reasons:
New cane swells each time exposed to moisture of playing
Blades not completely scraped or sanded
Staple too wide open or turned at angle inside the blades
Swelling with Moisture
This
is an otherwise well-made reed. The training procedure will handle this
reed just fine. Before you start a session, give the blades a pinch closed
at the upper part of the thick lower section to make it a bit easier. About
every 5 minutes you'll notice it getting harder again. Stop, take the chanter
out & pinch the reed again. Do this every time you find it getting
harder. One more time when putting away end of session. In all but very
dry climates it may be preferable to store the reed outside the bag, especially
for pipers playing every day.
Pinching also seems to sharpen those flat f's, which can be caused by
an oversized chamber between the blades just above the copper staple. This
naturally happens when a new reed swells in the moisture.
Another
trick is to fashion a metallic bridle or collar around the base of the
blades to hold them against swelling out of position as the moisture hits
them. Some pipers use elastic or tightly wound string, but these have a
disadvantage of continuing to pull even when the blades are in their ideal
geometry. A metallic bridle can be set slightly snug when the reed is dry
with its desired amount of elevation, and will of course strongly resist
further opening as the new reed swells with moisture. After the first 1/3
to 1/2 of the usual break-in period the bridle is removed. Usually Highland
pipe reeds do not play well with bridles, being prone to squeals and a
restricted sort of tone.
Incomplete Scraping & Sanding
You can't assume that the blades can be thinned in any one area, or
even uniformly all over. Other traits of the reed must be checked. A few
common cases:
Tuning,
tone, response generally OK to excellent -- scrape sides
High
G way sharp, top hand thin -- scrape upper center
High
A flat -- sand top 1/4" of lips lightly
Overall
way sharp -- thin the top of the hump or "tone box"
Staple Problems
It is possible that the top end or "eye" of the staple has
not been squeezed down as small as usual. Also the blades may have slid
around the staple as they were being tied on, a very common occurance.
In both cases this holds the lips more open which makes the reed strong,
and also makes the reed chamber larger than was assumed by the chanter
maker. The latter can lead to a flat F. If the cane is good such a reed
may easily be turned into an excellent reed.
In
a minimal case, a little more than the usual training may suffice. Beyond
this, especially if the staple is merely too opne, a slight squeezing of
the upper staple (with a protective tool such as an ice-pick, nail, or
specially-made reed mandrel inside to minimize the squeezing) should be
done to reduce the strength to normal starting strength. This should raise
the flat F noticeably.
If
the lips have slid around the staple, the reed should be retied. The blades
should be thoroughly wet, soaked for at least 10 minutes (as they are when
reeds are initially made), and the exposed part of the blades securely
wrapped with hemp or thread to prevent their slipping out of position.
(They will probably close; don't worry, they will return to an open configuration
shortly.)
The main binding is removed, and the blades are carefully repositioned
onto the staple. It is extremely important not to leave the staple farther
in or out of the blades. On tiny Highland chanter reeds, even 1/16"
change can throw notes out of tune.
The blades are then re-bound to the staple, very tightly with waxed
thread, hemp or dental floss. Each wrap must be snugged firmly against
the wrap below so as to make as airtight a binding as possible. The binding
must stop exactly at the top end of the staple, at which point the edges
of the blades must come together in an air- tight seal.
The remaining binding is then wound back down over the first layer (it
can be done in an open spiral) and tied off. The upper parts of the blades
are unbound to prevent permanent deformation and the reed is left to dry
for a day or so. The binding can be coated with varnish or nail polish
to seal it. The reed is now new and ready to break-in again.
This
is an otherwise well-made reed. The training procedure will handle this
reed just fine. Before you start a session, give the blades a pinch closed
at the upper part of the thick lower section to make it a bit easier. About
every 5 minutes you'll notice it getting harder again. Stop, take the chanter
out & pinch the reed again. Do this every time you find it getting
harder. One more time when putting away end of session. In all but very
dry climates it may be preferable to store the reed outside the bag, especially
for pipers playing every day. 
Another
trick is to fashion a metallic bridle or collar around the base of the
blades to hold them against swelling out of position as the moisture hits
them. Some pipers use elastic or tightly wound string, but these have a
disadvantage of continuing to pull even when the blades are in their ideal
geometry. A metallic bridle can be set slightly snug when the reed is dry
with its desired amount of elevation, and will of course strongly resist
further opening as the new reed swells with moisture. After the first 1/3
to 1/2 of the usual break-in period the bridle is removed. Usually Highland
pipe reeds do not play well with bridles, being prone to squeals and a
restricted sort of tone. 
Tuning,
tone, response generally OK to excellent -- scrape sides 
High
G way sharp, top hand thin -- scrape upper center
High
A flat -- sand top 1/4" of lips lightly
Overall
way sharp -- thin the top of the hump or "tone box"
In
a minimal case, a little more than the usual training may suffice. Beyond
this, especially if the staple is merely too opne, a slight squeezing of
the upper staple (with a protective tool such as an ice-pick, nail, or
specially-made reed mandrel inside to minimize the squeezing) should be
done to reduce the strength to normal starting strength. This should raise
the flat F noticeably. 
If
the lips have slid around the staple, the reed should be retied. The blades
should be thoroughly wet, soaked for at least 10 minutes (as they are when
reeds are initially made), and the exposed part of the blades securely
wrapped with hemp or thread to prevent their slipping out of position.
(They will probably close; don't worry, they will return to an open configuration
shortly.)