I mark in all but one of my Bibles, it helps me remember where different points are found. The following was written by DL Moody, hope you enjoy.
And
Bible-marking should be made the servant of memory; a few words will recall a whole sermon. It sharpens the memory, instead of blunting it, if properly done, because it gives prominence to certain things that catch the eye, which by constant reading you get to learn by heart. It helps you to locate texts. It saves preachers and class-leaders the trouble of writing out notes of their addresses. Once in the margin, always ready.
There is a danger, however, of overdoing a system of marking, and of making your marks more prominent than the Scripture itself. If the system is complicated it becomes a burden, and you are liable to get confused. It is easier to remember the texts than the meaning of your marks.
And
1. Scripture references. Opposite Gen. 1:1 write, "Through faith. Heb. 11:3," because there we read, "Through faith we understand that the worlds were framed by the word of God." Opposite Gen. 28:2 write, "An answer to prayer, Gen. 35:3." Opposite Matt. 6:33 write, "1 Kings 17:3" and "Luke 10:42," which give illustrations of seeking the kingdom of God first. Opposite Gen. 37:7 write, "Gen. 50:18," which gives the fulfillment of the dream. You can connect the prophets with the historical books, the epistles with the Acts, in this way.
2. Notes to recall a sermon, story, or hymn. Against Ps. 119:59,60, I have written, "The prodigal son's epitaph." This recalls John McNeill's sermon on those texts.
3. Railway connections; that is, connections made by fine lines running across the page. In Dan. 6, connect "will deliver" (v.16), "able to deliver" (v.20), and "hath delivered" (v.27). In Ps. 66, connect "Come and see" (v.5) with "come and hear" (v.16).
4. At the beginning of every book, a short summary of its contents, something like the summary given in some Bibles at the head of chapters.
5. Key-words for books and chapters. Genesis is the book of beginnings; Exodus, of redemption. The key-word of the first chapter of John is "receiving"; second chapter, "obedience"; and so on.
6. Any text that marks a religious crisis in life. I heard Mr. Meyer preach on 1 Cor. 1:9, and he asked his hearers to write in their Bibles that they were that day "called unto the fellowship of his Son Jesus Christ our Lord."