You may come up with the perfect idea. Then when you sit down to write, you don’t remember it. You need to keep up with your ideas. How can you do this?
There are various ways.

by Jubal Kenneth Bernal via Unsplash
Carry a notebook.
If you choose to do this, make sure it is a notebook that fits with your lifestyle.
Size matters. Don’t get a giant one if you need to carry it in your pocket.
Consider what it weighs also. I quit carrying one notebook because my purse got too heavy.
Use your phone.
I take notes on my phone.
I keep story ideas in one note. Otherwise I will miss/lose the ideas.
When I am walking the dog, I can whip out my phone and add something short onto the list. Maybe I will add to it when I get home. The most important thing is to get the idea down.
Do be careful that you add enough to jog your memory. I wrote “I don’t like x.” It was days before I could remember what I had been thinking and I may have lost some really good idea.
Use your computer.
Lots of people have a document or a spreadsheet where they put ideas.
I recommend a single document or spreadsheet or–like with the notes on my phone–these may be lost in the jungle of computerdom.
Name it the most obvious thing you can think of.
If you later think of some other obvious thing, add that to the page so that when you search using it, that will come up.
Also, if you have and use a computer, I recommend moving all the ideas from your notebook and your phone to this document.

by Kaitlyn Baker via Unsplash
I do that periodically, so that I have a complete list of the ideas I’ve come up with.
Once I use an idea, I don’t take it off. I just strike through it. Another author grays it out. Whatever you need to do, make sure you differentiate between an idea waiting to be used and one you have already used.
You may think you would not forget you’ve already used an idea, but you might. You might be able to use an idea twice if the uses were significantly different enough to make it worth your time.
So why not take them out? Sometimes keeping them in the order you got them will help spark new ideas.
Also, you can rearrange a list (by alphabetizing it) or rearrange a spreadsheet and get a new set of connections between ideas that you might not have come up with on your own.
Warning: As someone who has lost a computer, don’t just save it on your computer. Save it on your computer. Send it to yourself as an email. Put it in the cloud. Make sure you at least have two places/ways you can access this information. If it is the heart of your work, you need it to keep your work alive.
Never just save it to your work email. I once had an employer end my access to email the day I gave notice. Everyone else had given me long-term to in perpetuity access to my email, so I wasn’t expecting that and I lost a lot of personal data.