A few thoughts...
Do you incentivise your customers to promote you? A photographer I used sent me a voucher to give to a friend when she sent my pics. If you are new and don't have many customers, use a bit of time now developing a good system to keep records of your customers, so that you can encourage them to use you again. I use salesforce.com as my CRM.
Do you follow up enquiries? Busy mums (as you know) might just forget to call back.
Do you collect prospect details? Maybe do a prize draw to attract names/emails.
I volunteered for our NCT we offered various ways of promoting your business, each area is different. Don't just try the most local one, looks at neighbouring ones.
What about offering a group photo shoot for mother and baby friendship groups? You could target coffee shops and hand them a voucher? Offer to lay on cakes etc - make it a social engagement.
Schools often have opportunities throught the PTA.
Talk to a local kids clothes shop or toy shop, could you do something jointly?
Yes, give away a prize to the charity but ensure you get good brand exposure and it is something to tell your clients/prospects about & try to get press attention.
Are you adding a call to action? "10 shots for the price of 5, book by xx".
You need to be ruthless with the "fly posting", others are. Go to hospitals, maternity clinics, coffee shops, libraries, children's centres, softplay centres. Most of these have a place where you can leave leaflets. And go back, don't assume your pile will be there in 6 months etc.
Good luck.