| Man, I wish I was DMing right now. I check this topic periodically in the hopes that people post and describe skill challenges they've been running, but it's just dead.
I liked the question about "removing" the primary skill; it gave me several ideas. Sure, you could just reduce DCs by one to compensate, but I see several other options as well:
Ad Hoc: Give a +2 whenever they think of a really brilliant, effective tactic for the problem at hand, or when the description is heroic and awesome.
Behind-the-Scenes: Instead of 1 or 2 primary skills that the group is aware of, consider using 2 to 3 primary skills but don't tell the players which ones they are. It might encourage the players to think more about what their character would want to do, and less about how to take advantage of the known bonus.
First to Battle: If you have 6 or more players, or any group where die rolling and resolution takes a long time, you can encourage faster action by giving a +2 to the first two people who shout out their actions. | 
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