The Challenger shot could make it into the database with some processing. As you said you own a dslr, do you have a higher resolution file than the one linked above?
It's usually easiest to shoot at the biggest size, because correcting mistakes is easier with fewer loss of quality.
There's no real need to buy a ton of magazines and try to learn the technique from these, just look through the accepted shots in the database, and try to see what most have in common.
If shooting again, try to get the aircraft centered in your frame as good as you can, without cutting off parts of the plane (not like the NetJets shot).
Try to get the shot level, so that the verticals (e.g. lamp posts, buildings, etc.) are vertical. If there is no real reference, try to get the horizon straight.
With these things in mind the next time you'll be out shooting, you'll be on the right track for airliners.net acceptances, and I have to admit you're not doing too bad so far. Have seen much worse shots around here actually.