I have recently been working on new repertoire for my various gigs I have been playing and will continue to play this year. While I could spend my practice time writing new pieces, I find it not worth the effort lately when so many great pieces have been written. I recently discovered a work by a living guitar great Gary Ryan named "Hot Club Francais." It is truly a great work, which sounds like you are sitting in a café in Paris on a spring or summer afternoon. I received the transcription for my Birthday last week, and much to my surprise, I have the whole thing memorized.
While I understand not everyone can memorize music so easily, for some reason I have never had a problem with the process. There are a lot of musicians who stick to reading sheet music, and never memorize anything. While there is certainly nothing wrong with this approach, I find it easiest to memorize as many pieces as possible, particularly during wedding season, when any gust over 10 miles per hour can wreak havoc on your sheet music. I remember one time I was playing a piece of music at a wedding that I had not yet memorized, and I was near the Mississippi river, when a gust of wind ripped the sheet music off and would have gone down the river if it wasn't for the braveness of someone attending the wedding that chased it down. Luckily for me, my memory kicked in, and I was able to continue uninterrupted. This was my primary reason for trying my best to memorize every piece, no matter how long it took.
My approach is usually to listen to the piece of music as many times as possible to get a feel for what the composer had in mind, and start getting those notes stuck in my head (that is of course after I panic briefly looking at the notes, as is totally human and normal). From there I look at the score, and start putting the notes on the page in my head, doing a mental preview. Usually after a while I then pick up the guitar and start slowly, finding sections of the piece, and putting an alphabetic letter next to the section and really getting it down, until I can play it 5 times in a row without mistake. Next, I simple start playing the sections and repeating.
The brain needs rest to process what it has learned, so I usually do this for days at a time, depending on the difficulty of the piece, letting a few good nights sleep do the magic. I always make sure to listen to the piece of music and look at the sheet music again, until I'm confident I have every note down. The most rewarding part of this process is when you can play without the score anywhere near you. I'm sure everyone has a different approach, but this has worked wonders for me, as I have around 7 hours of repertoire memorized, and continue to add to the repertoire all the time. I like my approach, lest any wind gust ever take my sheet music for a ride!