At first glance there isn't a great deal which can be said about using the swell pedal. Isn't it the most obvious, the most straightforward, and the most instinctive part of playing the organ? Surely it requires no more skill than, say, using the accelerator of a car? Well no, that isn't quite true, especially if one aspires to perform pieces from the classical repertoire where a degree of ambipodiousness is called for.
It's different for those who have home electronic instruments with just the one octave of pedals. In that situation, there's a clear division of labour between the feet - the left plays the bass notes and the right operates the expression pedal, and for either to encroach on the other's territory would be unthinkable. The same is true to some extent when playing in a popular style with a full pedalboard, as in the theatre organ tradition. Maybe the right foot helps out a little, but it's usually the left which does the lion's share of the note playing, leaving the right free for expression pedal duty.
In the classical tradition of organ playing, the pedal part is likely to be more complex, more melodic and more legato - something which can only be realised by both feet working together in close co-operation. In this situation, the swell pedal needs to be operated by whichever foot can best be spared at the time. This means that a competent classical organist will be as happy using the left foot as the right, and able to swap them frequently when required. I must admit I don't find this at all easy. Somehow, it seems far more natural to have my right foot on the swell pedal. I have to concentrate quite hard to use the left one, otherwise my brain seems to lose track of which foot is which, with disastrous consequences!
This ambipodious quality I've described is greatly assisted by good console design. An obvious requirement is that expression pedal(s) should be mounted centrally. This can be taken for granted with modern consoles, but was not always the case. The Victorian pipe organ I learned on had the swell pedal tucked away in the RH corner making use of the left foot physically impossible. It should also be easy to find the swell pedal and to slide the foot on and off it. The common practice of putting the swell pedal in a little pocket surrounded by speaker grille is not good from this point of view. Many VPO setups are superior because they have the swell pedal bolted on top of a free-standing pedalboard with plenty of space all around it. One final ergonomic consideration is that there should be sufficient vertical space above the pedal. The knee is sure to rise higher when using the swell pedal than when playing notes on the pedalboard, so this should be the deciding factor when positioning the lowest manual. It should not be necessary to twist the leg sideways when the foot is on the pedal. Again, those of us who assemble our own VPO console have the advantage of being able to adjust this dimension to suit our leg length, unlike those buying an organ "off-the-shelf".
When it comes to the artistic aspects of swell pedal use, the best advice is probably to use the damn thing sparingly - incessant pumping is downright irritating! To create a truly impressive crescendo the opening of the swell box can be combined with the addition of stops. In this situation, I always think it's best to add stops to the swell department first, then open the swell box, as the effect is all the more dramatic with more stops drawn. The famous toccata from Boellman's' Gothic Suite is a good example of simple but effective swell pedal use. For a large part of the piece the swell box is firmly closed, then it is opened over the space of a few bars and remains open to the end. It couldn't possibly be simpler, but it makes a big impression, like lifting the lid off strongly felt emotions.
An organist also needs to develop a sixth sense to know where in its travel the swell pedal is currently located. There's nothing more annoying than peaking too soon! Theatre organs often provided a visual indicator of the current state of the expression pedals (a feature duplicated in Miditzer) and such an indicator would be a valuable addition to a classical organ's console, but I don't ever remember coming across one.