Firstly, I don't think there is a need to improve participation here anymore than with the Math and Chemistry sites.

I think the problem lies with the attitude of some people who started out thinking that Physics Stack Exchange would somehow be the next Math Overflow. Instead, it's ended up ticking over nicely just like other popular academic sites on the Stack Exchange network, open to a wide range of people including enthusiasts.

However, if you want to "improve" participation more towards Math Over Flow level, then alienate those at the lower level by excluding them from asking questions, and banning homework questions. That way you'll improve participation of professionals , while reducing [questions/day][1] and therefore overall participation: 

 - 2nd place Mathematics Q&A for people studying math at any level and professionals in related fields **494 questions/day**
 
 - 17th place MathOverflow
MathOverflow Q&A for professional mathematicians **42 questions/day**

Despite physics not being as popular as mathematics, it's still placed 9th, and I don't think there is a need to improve participation in its current form:

 - 9th place Physics Q&A for active researchers, academics and students of physics **51 questions/day**

  [1]: http://stackexchange.com/sites#questionsperday