How can I improve my concentration?

I have trouble keeping focus on small and repetitive tasks, or most things that disinterest me. I might also start daydreaming while doing something I typically enjoy (such as reading or watching a film) and find that I have to go back over whichever part I missed, sometimes several times.

I’ve recently been using a free trial of Lumosity but I’m not sure whether I’ll be forking out for the slightly pricey subscription any time soon.

Any ideas to improve my concentration?

Thanks in advance.