Back when I first got into Pokemon (my first game was Diamond version, despite the fact that i'm now 21) I didn't get the concept of levelling a full team of 6, I just focused all the experience of my Infernape so that by the time I got to the elite four he was in the 80's (still got his monkey ass handed to him by Garchomps earthquake mind) and just allowed the space fillers (usually starly, shinx and buizel) to take excess experience in double battles.
But since I started playing the games properly, at least in the main story I keep my team evenly levelled and try to focus on training new members as much as I can until they catch up with the rest.