Playing softly/clearly has always been a sign of whether I am practicing sensibly or not. As long as I warm up slowly and just don't jump in to the deep end of the pool, it works. The beginning of my warmup routine is only with breath attacks. You should not think about less air when playing softly. Take that big breath! If you are playing hard, make sure you warm down afterwards. Soft, long tones until your chops feel "relaxed" again.
Practicing softly is key to so many aspects of playing trumpet.
I teach breathing as follows:
1) stand up straight but not tense, feet shoulder width apart, head over the shoulders (here is a very good description with pictures:
http://www.monette.net/newsite/monette_concept_03.htm)
2) exhale as deeply as possible
3) inhale deeply and when you are full up IMMEDIATELY EXHALE(or play)
This transition between inhaling and exhaling is critical as many players tense up the throat at this point. Timing your breathing so that you are full exactly at the moment you want to start your sound is probably the most important lesson that you will ever get. - Do not tense up after inhaling.