Yes we use A5 post cards my advice would be............... target - target - target .
What you heard was correct people do hang on to them for longer but you must still repeat the process on a regular basis.
We obtain mailing lists from a professional company that ensures all the correct checks have been carried out such as mortascreen etc.
By target I mean ask them to provide you with a list in a particular post code or codes of all consumers in the ABC bracket aged 25 -60 (or what ever age your aiming at) then you can break it down and post out your cards on a weekly/monthly basis.
Take your time over the card content and you might want to put something on it with an offer that can be used while they have the card in their possession obviously this encourages them to hang on to it.If you send them another card with another offer on it in the course of your marketing then make sure on the new card it states, that this offer supersedes the previous one.
They work reasonably well but are just part of our marketing mix.