How to Request Another Recommendation from One Recommender
Sometimes, for one reason or another, your application to a school or program will fall through. As you move on with your career path and plans, you might find that you need a letter of recommendation from a recommender who helped you previously. There's no problem with that, although it takes a little finesse to avoid awkwardness.
A lot of recommenders will want to know whether or not your application was successful once you've applied to the program. If you don't make it in, if you change your mind, if you switch careers, or if something better comes along, you might find yourself unwilling to send out notifications that say as much. However, that's a great time to set the foundation. Even something as simple as, "Although I didn't get into the program this year, I'll be building up my [resume/financial aid/transcript] and re-applying next year. Would you be willing to keep my letter on file for resubmission in the fall?"
If you've fallen briefly out of touch with your recommenders, that's okay as well. Even a year or two shouldn't be a problem. Just get in contact with them a reasonable amount of time before the deadline and let them know what your new direction is. If the old recommendation still applies, the recommender might be able to reuse the previous version. Otherwise, fill them in on your new direction so they can provide an up-to-date recommendation.
Whatever you do, treat it like a normal request and don't be overly detailed or desperate. There's no shame in saying your last venture didn't work out and asking for assistance doing something new.
Index of letter of recommendation templates