The single most effective move is physical separation: put the phone in another room while you work. Willpower is no match for a device engineered to pull your attention, and even a phone lying face-down on the desk quietly drains focus because part of your mind stays aware of it.
Back that up by turning off non-essential notifications, using a focus or do-not-disturb mode, and keeping a notepad for the thoughts that make you reach for it. If you remember something urgent, write it down and deal with it later instead of unlocking the phone.