Using a Key is a great way to get creative using your own notation method.
Let me explain, a Key page is probably one of the first pages I create. Right next to an Index page. The Key catalogs the symbols you use, so you can quickly refer to them when jotting things down. This allows for me glance at pages and look for key points as needed. It also builds some consistency in your journal as you build it.
As you can see in the example, I have created a few symbols to denote various items.
An Open Circle is used to capture a Location, or Plot Point, name/title.
A check mark in the Open Circle tells me they have visited the Location, or the Plot Point is complete.
A line thru the Open Circle tells me the party has skipped the location and I may need to add it back in or return to it later in the campaign. Sometime I will use a highlighter to ensure I remember o go back to this location if it was important to the campaign storyline.
I also use the Hook, Line, and Sinker™ method for summarizing adventures in my campaign journal. I picked it up from Palladium Games, RIFTS series of adventures. Each step in the method gets a symbol. The Hook is obvious, gets a fishing hook symbol. The Line is a vertical line. The Sinker is a downward pointing arrow. This method allows me to quickly note something down during the planning/idea stage, and quickly denote it with the symbol that I think it addresses. This is especially helpful if the entries span more than one page!
The #(skill name or ability) key is a method to denote any major skill checks needed in an encounter. For example if the players are fighting a monster on or near a cliff, they will need to make a check using the climbing skill or relevant ability. The number is their target number and I note the skill needed next to it. Saves me time during an encounter, and is one less thing to think about.
Last, but not least, I use a letter surrounded by a square, to note order of events. This quickly allows me to track what order things need to happen in. This works best when you write your ideas into your journal and denote the order in which they make sense. Which is not necessarily the order in which they were written.
I hope you find this informative and I look forward to any Keys you come up with.