While Rowuk might be a bit hard in his approach, I think that the answer is pretty spot-on.
As a Freshman in HS I improved by leaps and bounds, and it wasn't necessarily because I was doing wonderful practice in the practice room. The short of it was that I had the horn on my face ALL THE TIME. Before school I'd show up early, run up to the band room, get my horn out and warm up for 15-20 minutes. At 8:15, first period of the day, we had concert band rehearsal. After lunch, I was back up in the band room with my horn out prior to jazz band and show choir, which alternated days (I was in both) so every other day I had jazz band rehearsal on top of the noodling after lunch. After school, I was back up in the band room, noodling around on the horn again. Then a couple of nights a week I had pep bands for volleyball and basketball games, and occasionally wrestling meets.
The key to our young poster is time. At this stage of the game there are no tricks - ImagineThat needs to put in the time - period. 30 minutes every day or so isn't going to do it. Then, it's going to boil down to just how badly they want it. For me it wasn't really a matter of wanting to be better than my peers - I was already better than my peers. For me, it was just the fact that I LOVED playing trumpet and couldn't imagine anything I'd rather do more with my time, hence the reason I ran up to the band room and got that horn in my hands at almost every available opportunity. We played some music that was challenging, and that didn't hurt, but I wasn't drilling daily out of Clarke or Arban's, that's for sure.
To ImagineThat, I'll simply say this - if you want to get better, then focus your efforts on what you want to be better at. Rome wasn't built in a day, and your chops won't be built in a day either. However, if you start putting that thing on your face at every available opportunity, it doesn't matter if you are running concert band tunes or pep band charts, you WILL improve.