I wish to add only a few points to the excellent post by raulb

What formation to fly?

I like to let both aircraft fly intersecating "snake" patterns (a
sequence of gentle L and R turns, in converging directions). There's
no danger: the pilot of the subject aircraft can easily fly towards
the tail of the camera-glider, passing just behind and keeping a very
wide control of the situation.
Staying very very close is no longer necessary with this pattern.
Gently banked gliders look much better, IMHO.

What is the best/easiest combination of distance between the aircraft and lens focal length?

Focal length of 135mm, or 80-200mm zoom, on the usual 35mm format. (or
equivalent). This is what I found to best suit my needs

Aldo