Hi Sheena
It can be tricky at first.  Like you, I am a trained mother-to-mother counsellor so I think that it is most important to validate what mothers are telling me, or the sensibleness of what they are trying to achieve (more sleep, bonding with dad, better weight gain, juggling working and mothering) and then to ask if anyone has ever talked to them about the risks associated with formula feeding.  (I also talk about whether there is any evidence to support their friend/mother/doctor's assertion that formula is the solution - babies wake, mothers wake when babies wake and don't usually trust dads, formula feeding requires much washing up and floor pacing between feeds.)  From there it is easy enough to let them know that babies who are fed formula are about 5 times more likely to be hospitalised in their first year of life - and that the more formula they get the higher the risk.   This usually starts a conversation about lifelong risks and how to weigh risks and how it is not possible to avoid all risks all the time.  If they have already exposed their babies to formula or the risks of not supplementing are higher than the risks of formula use, I talk to them about how they can mitigate the risks (meticulous sterilising, hand washing, staying away from sick people, using HOT water to make up bottles).  
IN promotion, I talk about breastfeeding being important or integral to babies' health and wellbeing.  I talk about the increased risks associated with formula feeding.   I work hard to adopt a professional (almost disinterested) tone - if X then Y, Y is important because X. If asked, (or a "my sister had to use formula because" is offered) I talk about how to decide if formula is necessary.  It is most important to avoid giving the impression that I have an opinion about how they should feed their babies.  I am there to provide information and then to leave parents to make their own choices.
Hope that helps
Nina Berry
Australia

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