〜ことにしている

1. 1Indicates a regular practice one has chosen to keep: "make it a rule to do."Ex. 1
2. 2With the negative form, indicates a self-imposed rule of avoiding something: "make it a rule not to do."Ex. 2

Used in everyday speech and writing for habits that come from a conscious choice, often with a stated reason such as 健康のために. It is the continuing state of ことにする: the decision was made earlier and is still being kept. The decider is usually the speaker, but an organization can also use it for a policy it has chosen. It differs from ことになっている, which presents a rule as set by others or by circumstances.

• ことにするN4ことにする focuses on the moment of making the decision (ことにした, ことにします), while ことにしている focuses on the habit or arrangement you keep up after deciding, so the two cannot be freely swapped. ことにする also has the sense of deciding to treat something as ~.
• ようにするN4ようにする stresses being mindful and trying your best to do something, while this pattern stresses following a rule you have set for yourself; both can describe habits, but with different nuances. ようにする can also mean managing to bring about a certain state, while this pattern can also express a future plan that has already been decided.
• ことになるN4ことになる describes arrangements made by other people, an organization, or circumstances (it can also state your own decision in an indirect way), and it can also express a result reached by calculation. ことにしている instead highlights something you decided yourself and keep doing, so the two are not interchangeable.
• ようになるN4ようになる describes an ability or habit gradually changing over time (often as ようになった) and does not emphasize willpower, while ことにしている stresses sticking to your own decision, so the two are not interchangeable.
• ものだN2ものだ, attached to the た form, recalls things you used to do often or expresses amazement at how things have changed, while ことにしている is about habits or arrangements you set for yourself and keep up. ものだ also gives advice based on social common sense rather than personal decision, so the two are not interchangeable.
• つもりN5つもり expresses your own intention and cannot describe a habit or rule you keep up consistently; the two are close when talking about future plans, but ことにしている stresses that the decision has already been made and is being maintained. つもり can also mean 'under the impression that ~' (believing something that is not actually so).

ことにしている is the ongoing result of ことにする (deciding for oneself), unlike ことになる (a decision made by others or by circumstances). ようにする means making an effort toward something, ようになる describes a change that has come about, ものだ states general truths or, as 〜たものだ, past habits, and つもり states an intention, not an established routine.