Never leave milk leftovers in bottles, but throw them away immediately after feeding. Milk residue attracts bacteria which can cause food poisoning and diarrhoea.
Wash bottles and teats immediately after feeding, don't leave the dirty dishes in the sink. Later on it's very difficult to get rid of dried up food that sticks into the bottle and teat. If you don't have energy to wash the bottle after night feeding, at least rinse the bottle and fill it with water to prevent traces of milk from drying in it. Wash the bottle thoroughly in the morning. If you leave bottle washing for the last moment, the baby might be demanding for food and you won't have any clean bottles left. Before you manage to wash one thoroughly, sterilise it and fill it with food, the distressed child will be crying at the top of his voice.
Don't use any caustic or strong cleaning agents when washing the bottles. Warm water with washing up liquid or baking soda solution is enough. Wash the bottles thoroughly with a bottle brush.
Make sure that no food leftovers were left in bottle cavities, then thoroughly rinse the teat and bottle under running water. Wash the brush after cleaning as traces of food might attract bacteria. Some bottles and teats can be washed in a dishwasher, but remember to check the producer's recommendations in the package leaflet before washing. After washing, place bottles upside down and let them dry, or put them in a steriliser. Do not wipe them.