generally playing against people who are slightly better than you will cause you to improve the fastest
scrims are the best way to improve followed by pickups
demo reviews are good for every class - although it is better to get a mentor at lower levels than to review them yourself imo
dm is good up until a certain skill level (right around mid IM I think) at which point you might as well be playing a pub because basically everyone you are playing with is worse than you
if you can play mge against someone you find a good match for you whether it be scout v soldier or scout v scout I think that is a lot better than playing DM
playing mge in general has a lot of dead time so I would say dm is better than mge when you are playing with randoms
generally you want to be INGAME (scrimming or pugging) for at least 2.5 hours if you really really want to get better and that's really the important part