I can’t find my citations-- I think they were Gertrude, Agreda, and Liguori, respectively, but I could be wrong–
You can imagine that your prayers-- petition, adoration, contrition, thanksgiving-- are like a treasure chest. If you say a good, thoughtful prayer full of love, you can imagine that it’s like a diamond that goes into the treasure chest. If you don’t… well, they’re ordinary rocks.

By the end of your lifetime, how full is your treasure chest?
31 “Or suppose a king is about to go to war against another king. Won’t he first sit down and consider whether he is able with ten thousand men to oppose the one coming against him with twenty thousand? 32 If he is not able, he will send a delegation while the other is still a long way off and will ask for terms of peace.
The king with 20,000 men is God, and the king with 10,000 men is you. Maintaining the prayers = treasure symbolism, imagine that you must pay tribute to the stronger king. Will the stronger king ever tell you, “You’ve paid me enough?”
You can imagine that your prayers are each part of a feast that you’ve put together to honor the King. If you are attentive, it’s a magnificent feast, where the parts all work together to make a magnificent whole. If you’re inattentive— yeah.
One problem I find is that it can be hard to focus. What was recommended to me was to imagine yourself lying underneath a river. You see the boats passing above your head. The boats have your thoughts on them. If you have a distracting thought, you let it pass down the river and out of sight, and continue to focus on your prayers.
Another difficulty is that the repetition lends itself to meditation… but often I find myself meditating on my to-do list, my Christmas list, my grocery list…

How many layers of thought can you keep going concurrently? Give each level of your attention something to focus on. Say your prayers with your voice. Pay attention to the meaning of your prayers with one level of thought. Contemplate the mysteries with another level of thought-- walk through them as though you’re living them. There are some meditations that offer a different meditation for each prayer within the decade-- give that a layer of thought, too. If you have spare levels of thought, use one of them to contemplate your intentions, or the people you wish to help through your intercession, or praise or thanksgiving or honor. And so on.
Good luck.