The old books talk about making a 'bee bob'.
 
Take something like an old sock, dark coloured, stuffed with anything you
like (other old socks perhaps). Get it thoroughly impregnated with old wax
from brood frames, then tie it up in a tree. You can also nail it to a piece
of wood (the size of a hive roof) which can be easily removed. This can then
be placed directly over a new hive, with one brood box of frames, and an
empty to allow space for the swarm cluster.
 
The idea is that the bees are attracted to it for two reasons.
1  They think it is the beginnings of the swarm cluster, because of its
appearance.
 
2  They love the smell of old brood frames (wax, propolis, pollen, etc.) and
that attracts them too.
 
These methods are said to be especially effective if you know of sites that
are often frequented by swarms.
 
By the same principle a couple of old brood frames (not honey frames) are
supposed to work best when left in a box or hive as bait.
 
[ Having said which I have not personally tried these techniques, but plan
to this year.]
 
My father has tried out the lemon grass (lemon balm over here) but said it
made no difference.
 
Madeleine Pym, London, UK