William Quixote:
Julian,
I got the technique from Leigh Brasington. Basically, you follow the breath until your concentration is warmed up and you are really on the breath. Once you feel like your concentration is strong you smile and hold it. Once you do that, you should feel a warm positive feeling that's triggered by / associated with smiling. Then, you keep following the breath, but put your mind on the pleasant feeling associated with smiling. Its partially physical and near the smile, but its more mental. Its the part of the feeling that lets you feel that a pleasant feeling is pleasant. Then you kind of hold that in your mind without being distracted or wanting it and it gradually builds. Then at some point you relax into the feeling and are in first Jhanna. It took a while to get the hang of, but I think its a pretty fast way into first Jhanna once you've got the trick of it.
As a further reference, Leigh Brasington teaches a lot of retreats focused on Jhanna and has a website with a lot of good content on Jhannas http://www.leighb.com/jhanas.htm and talks on Jhannas http://www.leighb.com/talks.htm.
Kind of a late reply, though thanks a lot for explaining all this in detail!