What is the best way to use postcards in advertising one's business ? I don't know anything about this topic and on Google I only find stuff about how to actually design postcards, but nothing about how they are used in advertising ..
Being a graphic designer moreso than a Web Designer I've made postcard designs for small businesses over the years, most of whom simply put them up in the local Post Office / shop window. Because they are 300DPI (mine anyway) they can also be used in press advertisements.
Don't take this the wrong way... but why on earth would you consider using postcards for advertising a business?
Even if you're not a technology business I can't really see a practical use for them apart from as yet more junk mail.
I don't disagree with using postcards, but maybe you should rectify your use in them. A postcard is something that people normally send on holiday, and companies send them during the holiday season to their existing customers, like a "hello, we still exist", meaning.
If you wish to send postcards maybe you should send them to existing clients during the holiday season, so Christmas and Easter. I would not send a postcard to somebody you don't know, as it's likely to be ignored. This would be advertising to existing clients.
Hope this helps.