although there is no accepted/practised way to get an idea doing these particular things wont harm in the genesis of an idea.
1. Look around. Carry a book and a pen always. Note down whatever you see and whatever you can't. Relate the incident to something you knew, felt or heard.
2. Read more about a thing you want to write. If you are not sure what to write then write about why you are not sure and what makes you think a particular idea was not worth writing about, That would give you more stuff to think and write about.
3. Write more. The more you write the better your ideas will be since the inferior ideas were already out of your system, so there is only place for better ones.
4. Travel by Public Transport and attend any government organization like hospital, RTO Office etc...This should give you a lot of stuff to write.
5. Always try to know why you felt the way you felt about anything and everything.
6. Last but not the least, don't think that a fantastic idea comes just like that. It will more often than not be a result of a 1000 mediocre ideas. The more ideas you reject, the better your quality of work will be.