I'm using an interdental brush.

This is the I-type interdental brush I've been using lately. I tried several different types before finding this one at a large supermarket, so I've been using it ever since. It's easy to use and removes food debris between teeth, so they say it's good for preventing plaque and tartar. It also seems to have a nice gum massage effect, so I make sure to use it after brushing. I think it's good to choose a type that suits you. It comes with a hygienic cap, making it convenient to carry around, so I always take it with me.