You can go into any petstore and pick up some doggie toothpaste. It dosen't take too long to get them used to the teeth brushing. Start out with your fingers first with the toothpaste and slowly build up to using the toothbrush.

There are fresh breath biscuits, if you're interested in trying. Pedigree has a line of Breath Buster Biscuits...it has parsley and eucalyptus in it so, like wolfsoul said, you could try those.

How old is Marta? I second LoudLou that you may need to get her teeth looked at..maybe one is rotted. You could also have the vet do the initial dental work, with scraping and all that, and then start on a routine of teeth brushing. Hope that helps!