What can I do to improve my concentration immediately?

Learn some meditation techniques. Use some imagination exercises. You focus on your breath or on something else, and then you deliberately bring in some imagery that relaxes you and focuses you on what you want to focus on. These techniques can also deal with fear and anxiety and other things that block focus.

Stimulants help.

Working at a table with no distractions helps.

This is not an immediate fix, but I know that there a lot of summer getaways that teach the fundamentals of focus. They are usually group efforts that take place in the wilderness on a lake.

I think they are called Concentration Camps.

incendiary_dan is correct. Relaxation is not what you’re going for. Drink a cup of coffee or take NoDoz.

New answer.

Tell a good friend to tell your other friends to slap you in the face when you’re not expecting it. It’s immediate and you’ll be on red alert.

Drink a glass of warm clean water.