Give your brain a rest stop 

At last!  The advice you've always wanted to hear.

Your mind wanders.  That is incurably normal.  It is what brains do.  You can fight it.  Or you can use it.  Here's how to use it.

If you want to fight it, you’ll have to work that out on your own.

When you notice that your mind has wandered, write down where your mind took you.  Then take a break. 

If your mind wanders, follow it.  It may know something you don’t.

If you are doing something that takes brainpower, don’t try to work more than thirty minutes at a time.  If you really get focused, you will work a lot longer.  But you won’t have to try.

It is not that your brain gets tired.  It gets focused.  It does not need a rest.  It needs to hear from some of its parts that got pushed into the background.  To make that happen you need to break focus.  Make notes or marks to remind you of where your were.  Then relax and take stock of what you have accomplished. 

Another name for focus is tunnel vision.

Are you really making the progress you expected?  If so, you may want to get back on the project while you are in the groove. 

If you are not making the progress you expected, you probably will do better to knock off the job for a while.  Because there is a difference between a groove and a rut.  If ideas pop into your head while you are doing something different, write them down.  Psychologists call this incubation.  Some parts of your brain  can’t get through to you while you are focused.  You have to take a break to hear from them. 

Taking a break is as much a waste of time as sleeping.

When you come back to your task, clear your head.  Don’t be surprised if you turn up some more ideas.

If you don't give your brain a break,
it can be creative without ever telling you.

The other reason for checkpoints.
Check points

Relax

Incubation

Chop big jobs into joblets.