Where you would like to mount it is important.

For external mounts, the ideal system uses a fabricated glass cuff
that is conformal to the chosen surface. The cuff is then taped to
the wing and a ball camera mount mechanically fastened to the cuff.
A crude but highly effective wing/horizontal stab mount can be
fabricated using a block of foam, sandpaper and some high quality duct
tape. The foam is contoured to the stab/wing shape, the block is
taped to the surface and finally, the camera to the foam.
Alternately, a plate of glass, metal, or thin wood can be taped/bonded
to the foam and the camera attached typically via a 1/4-20 screw. This
allows some added flexibility in pointing the camera. A ball mount
can be attached to the block to allow the ultimate in aiming.

Some people use a section of PVC pipe to hold the camera outside the
cockpit in order to to shoot back at the aircraft and pilot(s). Many
dramatic images have been captured this way but the work load in a
single place ship is markedly increased, thus increasing risk.

Internally, the camera can be clamped to the forward carry-through
tube using commercially available mounts such as the RAM series or
similar camera mounts. This allows a 3/4 forward view and is
adjustable on the ground but not in the air without additional
mechanisms and possibly remote viewing screens.

Mechanical Safety issues related to internal mounts include: Being
struck in the head with an XX kg mass traveling at XX km/h should you
crash while it is installed. I have hear that 10 "G"'s is a good
number for items in the cockpit. Should the camera become unattached,
it must not be able to jam controls or jam /break the canopy. These
issues suggest that a sturdy (10+ "G") secondary leash also be
attached.

Mechanical safety issues related to external mounts include: aft cg
conditions, reduced aerodynamic control effectiveness, lateral
instability due to the camera's mass on one wing, damage to the
primary structure or controls should the camera become partially
unattached from the surface and repeatedly impact the glider, and
jamming of flight controls for the same reason.

Hand-held video by a single pilot is the worst safety-wise as it
combines the worst of all possibilities: The camera is in the field
of view, potentially obstructing vision and especially binocular
vision, and the camera is loose, representing a projectile in the
cockpit that should (must?) be secured.

Be aware that attaching an object to the exterior of the glider will
likely invalidate your airworthiness cert. and thus your insurance
coverage, especially if the ship is standard airworthiness.

Please recognize that aerial photography is incredibly demanding of
the pilot's attention and thus adds a significant workload. This is
especially true if the pilot is shooting video, thermaling in
proximity to other ships or terrain, and providing on-camera
narration. Something is definitely going to suffer, and humans being
what they are, image capture will inevitably take precedence over
pilotage, terrain avoidance, and traffic scans.

Based on extensive personal experience and the multitude of crashes
the military has experienced doing air-to-air photography, I believe
that such undertakings are inherently very dangerous and require a
good plan, a briefing for normal and emergency conditions, and
experienced heads-up pilots, especially if 2 single-place ships are
involved, and the camera is being actively operated by the pilot. In
this condition, flying the aircraft must be second nature.

Unilaterally performing aerial photography of another ship without all
of the above is very poor form and especially dangerous.

Finally, try not to record FAR violations and poor pilotage (e.g.,
hand-held video showing the pilot not scanning for traffic or
departing controlled flight are the wrotst I have observed to date),
especially if you plan to put it up on the Web. Footage on the Web
should be considered a permanent record and it will be used with
maximum prejudice against those involved as well as against the
greater soaring community by the FAA, the insurance carrier, the
grieving widows, and the public should something go poorly in the
future. Lots of this already exists and we should avoid adding
more.

Be Safe and get some good images - but please do it in that order!