I used to use training methods to try and minimize stretch going into flower, but it seems it is as easy as light spectrum
You can either A - gradually lower the lighting hours from 18/6 by an hour every day until you get to 12/12 (mildly effective)
or
B - Instead of jumping from 6500K MH to 3000K or 2000K HPS... add in a 4000K from 12/12 flip until end of stretch (very very effective)
I had a bit of play with different bulbs a few years ago and was turned off by a 10 000K bulb that made my plants finish way too quick!
One of my peers got me onto Solistek's new range of bulbs which I found work fantastic (not a plug ^^)
He showed me the difference from going from 6500K MH bulb in veg to 4000K bulb from flip to end of stretch... and then onto 2000K HPS for the rest of flower.
I was quite amazed at the difference it made, especially with his sativas which now form single, long donkey dick cola's instead of popcorny smaller buds at the bottoms until the main colas start to beef up after stretch and close the internode gap.
Also a reduced stretch of up to 50% which can be useful with the usual 8 foot finished sativas.
I wouldnt really ever recommend people spending money on unnecessary extra bulbs.. but I see it work over and over again.
Cheers Jimmy.