As an aside: I thought tanks were characters? Or, at least, a party role.
I suggested that as well, using a tank figure to represent a heavy fighter. He didn't like it.
At this point though I really don't care. He and I are in agreement half the time, and I'm guessing that that's as good as it's going to get. Soooo...
Back to the original question: Becoming proficient in character design and implementation.
Multiclass carefully if at all, at least at first. Some classes complement each other and build towards a coherent whole. Others don't.
For example... Adding a single level of a spell casting class to a fighter can give a valuable edge. Adding a single level of a fighter class to a spell caster, however, usually isn't.
Some that you'd think would work well together, don't. Ranger and Druid, for example. They're each woodlands based, conceptually, and Rangers eventually gain access to a subset of the Druid's spells. But that's actually the problem, since the mixture just adds more of the same thing you already have.
Over all there are two basic approaches to character design: One is
intensive, focusing on a relatively narrow aspect of the game and mastering it. The other is
extensive, characters who are competent in a broad array of areas, but aren't top of their field in anything.
Characters will usually fit somewhere near the middle of this scale, with extremes being rare. You can design a character who is a holy terror in melee, all but unstoppable, but are medium-useless in non-combat scenarios, and only slightly useful in ranged combat scenes. (think of the mighty warrior with his +37 Blade of Awesomness, and a plus-nothing light crossbow that he usually fires once, then throws rather than take the time to reload, because it does more damage that way.)
Decide what the focus of the character will be, then decide how tight you want that focus to be. Then build towards that goal.