I actually never had a problem with this back then because I always took my 3DS when I went out. I never had to shake it to get play coins. Now, I don't really go out anymore, but I've still got plenty leftover from before
Oh man, how many do you have now? Approximately. XD
Right now, I have 129 Just take your DS with you! It'll accumulate naturally and you won't have to keep shaking it ^_^
I have literally spent the last ten minutes shaking my 3DS to get play coins to unlock stuff in Happy Home Designer. I feel so dumb. Nobody likes play coins.
OH MY GOODNESS YES!
Where I live nobody takes their DS with them when to go out (even when the system was new). That's why streetpass never worked too, except with friends with a DS. So I have spent quite some time shaking that thing sitting at home. ANNOYING!
Just remember you can only earn 10 a day so dont go on a marathon hoping to earn 1000 at once.