Everyone's probably had their SAC by now, but I thought I'd pitch in with a slightly different way of looking at soft power... achre's obviously explained it really well, but I think another way at looking at soft power is as the 'PR' of a state.
Soft power is everything a state does to make itself look attractive to other states; Nye refers to this as the "power of seduction". If a state's culture, political values and foreign policy are attractive, other states will want to emulate it - this creates natural allies, and, like achre said, makes other states more likely to align themselves with the 'seductress' state's national interests.
So if you're evaluating the use of soft power, just ask yourself 'does this action make *state* look attractive to other states', and don't forget that soft power doesn't exist in a vacuum - more often than not, it's undermined by aggressive uses of hard power (well, at least in China's case). You can see this in the negative reaction to Confucius Institutes - China's aggressive propaganda and repression at home means the spread of CIs has been accompanied with paranoia about China trying to influence the discussion about itself in universities and schools. I'm going off-topic but basically China can't get out of the mentality of carrots and sticks, so it can't use soft power effectively. How good can China really look when it gives $100,000 in aid to the Phillipines after Haiyan because it's angry at them about Scarborough shoal while IKEA gives $1.6 million?? I think I'm just trying to procrastinate, sorry.
So yeah, idk if helpful, but maybe a different way to look at it? Soft power = looking sexy.