Suggestions from my college professor:
Write for 15 min every day. Yes. Every day. Eavesdrop on people, or write your train of thought. Just write. Even if all you write is "I was here" or start doodling, that's acceptable.
Now some expanded tips I've taken from my prof to create an education class for a writing marathon (2hrs long):
Round 1: 1 minutes
“Pick up your pen and listen to your train of thought. The goal is to write everything your train of thought is saying: “I'm hungry, I forgot my shoes, I need to get in line for that panel after this, I hope my fish will be fed.” You will be writing for 1 minute, non-stop.”
Round 2: 1 minutes
“This time you're going to start with your pen down on the table in front of you. You'll listen to your train of thought, and wait for something interesting to pop in, pick up your pen and write that thought down, then put the pen back down and wait for the next interesting thought.”
Then you alternate between other things, prompts and no prompts, at varying times. Such as:
Round 3: no prompt, write for 3 minutes.
Round 4: prompt “stolen candy bar”, write for 3 minutes.
Round 5: prompt “lost island”, write for 5 minutes.
Round 6: no prompt, 4 minutes.
Round 7: no prompt, 5 minutes.
Round 8: prompt “favorite snack”, 5 minutes.
Round 9: prompt “flowers in winter”, 3 minutes.