Well, I’m sure you know there are “many” things you can do.. but if you insist on having just “one” thing.. well then.. your answer is in the following Hadith:
Sufyan ibn Abdullah reported:
“I said: O Messenger of Allah, tell me something of Islam which I will not ask anyone else about it. He SAW said: Say ‘I believe in Allah’ and then be steadfast” [Muslim]..
Having said so, I wrote this a couple of years ago.. and I think it helps one being steadfast.. so you might give it a try after abiding by the prophet’s advice above:
The Quran is all white, whereas everything else in this world is a shade of gray. Read it daily to cleanse your hearts and purify your souls.
And Allah knows best..