Actually, DON'T put flyers in mailboxes.....it's a federal offense to use a mailbox for anything other than postage paid mail. According to our postmaster, don't even tape things to the outside of mailboxes.
Networking groups can be a good start....also join your local chamber of commerce.
Advertising can be a super huge expense, and honestly, I think it just takes trial and error to see what works best for your type of business. Any type of glossy "magazine" style ads are very expensive, as can be newspaper ads. Having a website can be very inexpensive over the long haul and your best marketing bang for the buck. Just make sure that if you create it yourself that you submit it to search engines so that it can be found easily when people are searching for you.
Good luck, and just keep in mind that most businesses really don't get off their feet until well into their 2 or 3rd year in business. So, don't get discouraged, just keep working at it!