We have three cats and a dog. I try and brush everyone's teeth two to three times a week. The dog and two cats are easy and I can use a finger brush or toothbrush on all of them. I use a kids soft brush on two of the cats. The third can is a little trickier. For her I usually just put some on my finger and smear it as best I can on her teeth/gums or just outside and she licks it off. I figure at least that way it does a little.
To get them used to it, make sure you have a flavor of pet toothpaste that they like- beef, seafood, fish, and poultry are all options and I'm sure there are more. Start with a little dab for them to taste on your finer or just smear a little around their mouths for them to taste or lick. Gradually over a few days if needed left the mouth a little and brush or use a finger and swipe the teeth. For the cats the finger brush is really too big. With a regular toothbrush I can just lift the skin a little and brush or can open their mouth by pushing in gently at the jaw where their mouth hinges open and I can brush. The two that like it lick it off usually as I am brushing and then I usually let them clean the toothbrush a little more when I am done.
There are also CET treats that are great and mouth rinses that are sometimes easier. I believe CET says it is good to chew, better to rinse and best to brush. I give our cats a CET treat usually after I brush and the one that doesn't like her teeth brushed sometimes just gets the treat if I can't get her to cooperate. I can force her and get it done, but usually it is not worth it since she will bite and scratch for blood if it is not her idea. Wrapping in a towel works to keep the claws at bay if necessary. Her problem is more just a general restraint then the teeth brushing, so we brush where we can and treats regardless.