I use a baby bottle cleaning brush I've had a long time, it has a very narrow metal spiral that will go into most tiny openings. I also have a dog toothbrush that has an extra long handle (never could figure out why) that works. Put warm water with baking soda in the bottle and let it soak a while first, then use the brush. The baking soda is slightly abrasive and also sweetens the jar. For hummingbird feeders, after I use the toothbrush, I put more warm water in plus just a drop of bleach when cleaning before storing away for the winter. Rinse thoroughly and let dry before storing away.