This is a simple guide, mostly the build, for people who understand his abilities.
Yeah, I know I'm lazy.

1st build is for 3v3, 2nd is for 5v5.
General Items: I forgo AP items in favor of heavier tank, and eventually a frozen mallet (rather than a Rylai's, replace with Trin Force or Rylai's if compulsed) for more reliable and sustained slow.
Runes: I use attack speed marks, flat armor seals, MR/level glyphs, and move speed quints. Yeah, move speed quints. I prefer the increased move speed instead of, for example, increased attack speed, because Da Bear isn't a very effective tank if he can't get in on the victim. Matter of personal preference, and my preference is the extra security combined with Q and Phage.
Masteries: I go 0/21/9, basically only switching the tier 1 mr in defence for minion defence (Jungling is just easier) from 3v3 to 5v5.
Skill Order: Pretty simple,
3v3 = E first, max E first, W at 2, Q at 3.
5v5 = W into E, into more W, split at 7, maxing E first.

3x3. Open with a Doran's Shield (Null-Magic Mantle if the opposition is AP-heavy), Merc Treads, rush a Warmog's or slip in a HoG (om nom nom g/10). The rest explains itself.
5x5. Item-wise, I play Da Bear more supporty tank than straight-up tank. That said, start with cloth/pots, Aegis, Merc Treads (about when you should start ganking hard), and after that you can build your mandatory Warmog's or move the phage up in the order to ease ganking. Stark's is only relevant if your team has 2 or more AS champs (ironically, I got the idea from a Bear who bought it for himself, with an Atma's [Didn't work out for him, so I don't recommend]), otherwise Sunfire Cape, or a Rylai's to cement your hold on the slow-market.

Q is not for running away.

Comments appreciated, 'specially since this might become a full guide in the future (but with the jungle changes, who needs a 14,000 word guide any more?)