I recently began teaching the girls to count by 2's, 5's & 10's using a method I fully and happily exploited in grad school: the more senses you can employ when learning something new makes the material "stick" a lot better than just one-dimensional learning from a book or a chart, which is part of the reason aromatherapy works when it comes to memory.
I started by combining their PE lesson with math by doing a series of literal exercises with them and counting in this new pattern as we went along; we did sets of jumping jacks, squats and push-ups. Then we followed up with some good 'ol book learnin' using drawing and manipulatives (hands-on objects they could physically count with)- we used some little wooden alphabet building blocks I happened to find weeks prior at the Thrift store, but beads and buttons work just as well.
For the drawing portion of our learning, we took two exercises from the book I always rave about (Learn-At-Home,1st grade) where, for counting by 2's I had my 5-year-old draw 5 stick people with two arms in the air and count/add up their arms, writing the corresponding number above their heads (2, 4, 6, 8, 10), then for 5's I had her trace her hand ten times on a large sheet of construction paper and then counting/adding up the fingers, write the corresponding number on the "palm" of the hand (5, 10, 20, 30, 40, 50).
My 3-year-old was great with getting her teeth brushed up until just a few months ago, when all of a sudden it became dreaded and unbearable (for the both of us!) for some mysterious reason! So, after a few days of what felt like pulling teeth instead of just brushing them, it occurred to me that I should try the counting we'd been learning to get and keep her attention so we could get through the teeth-brushing sans missed spots, cavities and tears. I was allowed to brush hassle-free and the deal was I'd let her spit once we got to our top number (10, 20, 100), so at least she knew exactly when it'd be all over and that seemed to make her more willing to wait and listen to the counting. Let me say, it worked beautifully!
Now, every time I brush her teeth, I count by 2's to 20 (nowadays it's up to 50 so she can get a true sense of the pattern), by 5's to 100, or by 10's to 100, I usually mix it up daily. It's even carried over to my 5-year-old- she enjoys hearing me count, too and it's great because now I've got a great, functional way to reinforce the numbers and patterns without it being boring or grueling to them. Where would I be without these flashes of inspiration?!?!