I'm about 3 months behind on Project Life. And from what I read around the web it's a common 'issue' with Project Life...falling behind. It's easy to do...life gets busy and before you know it several weeks have gone by. And that's when some people give up. But I think PL is such a great way of recording memories that it's worthwhile to continue.
So here's what I do to catch up.
Photo editing & organizing
This, to me, is by far the most time consuming part of PL. I know there's some people have such beautiful, professional looking photos in their albums. And that's great for them. For me, I simply want them to look good. They don't have to be perfect and, in fact, sometimes I use photos that are maybe out of focus or too dark because they convey the story that I'm trying to tell by using that photo.
Bottom line...I don't stress over the editing part.
What I do like to do when I'm behind on PL is to add journaling directly on the photos in Photoshop.

This saves time when I'm putting the photos in my album & I'm not stressed about 'dressing up' my pages. To me, getting the photos in the album is more important that all of the extras like embellishments.
I organize my PL photos by month and week on my computer. Again, this helps me organize the photos for my album. I also rename the photos by the date when I save them in that folder. That way, when you get your photos back from the printer it will again help in organizing the photos for your album. Then I just stack them by week so I can add them to my page protectors.


Planning your layouts

This is not something I always do if I'm just doing a week or 2 of PL. But when I'm behind I find this to be a big help.
Some people sketch it out in Photoshop. That seems alittle time consuming to me. Not to mention, I have a bit of carpal tunnel so the less I can do on my computer the better for my wrist :) I've also seen some people download sketches but I just draw them out on my own because sometimes I'm going back & forth between different size page protectors depending on the photos I have for that week.

When I got
my photos back from the printer I simply organized them by week &
slipped them into the page protectors. Having most of the weeks already sketched out helped tremendously!
Keep track

Another tool that helps me is jotting down events & things I want to remember in my calendar.
When I'm going through my photos & sorting them week by week it helps to have my notes of what happened that week & what I want to remember such as soccer & baseball games & practices, movies we went to, company in town, places we went, etc. I will forget this stuff if I don't write it down. Especially if I'm going back 3 months.
This is also helpful in keeping track of what my little one is doing/saying/learning. Again, as much as I don't want to I will forget if it's not written down.
Focus on what's important
Finally, as I said earlier, when I'm behind on PL I don't worry about all the 'extras'. My main goal is to get these photos in the album. All of the extra embellishing can come later...or it may not come at all. And that's ok.
When my kids look through my scrapbooks, they're not commenting on what kind of ribbon I used, or the embellishments. They're looking at the photos & the stories they tell.

For now I simply have them organized in the album and I used my date stamp for each week. I got mine in the first edition of PL but you could easily get one at an office supply store.
When I go back to fill in the pages with journaling/etc I may decide to change out the title page, I may not.
Bottom line
Getting caught up is not as daunting as it seems if you simplify the process.
--> I took 2-3 days to photo edit, organize my photos on my computer, upload them to be printed, & draw sketches of my page layouts.
--> When the photos came in the mail, I took one night to organize them & pull out my album, core kit, stamp pad & date stamp, pen & page protectors.
--> I took one night to sit & put all of my photos in my album.
--> And over the next few nights I'll sit & fill out some journaling cards for the pages.
Most of this was done at night after my kids went to bed so I could spread out on the kitchen table without anyone bugging me. And I got caught up in less than a week!
So it doesn't matter how far behind you are. Believe me, I'm behind on the last 2 years of PL, not to mention my little one's baby album :) But you can get caught up, just find what works for you. Maybe weekly layouts don't work for you...then do a month at a glance. Maybe the 12x12 size doesn't work...do what my friend Virginia is doing and create a smaller album. Or you could even jump around & just do a week here, a week there. No one says your album has to contain every.single.week of the year!
Point is, we shouldn't get so caught up how we're going to record memories to the point that we don't at all!