So you have a lot of videos you need to go through to figure out which ones to included in your new show reel.  Question is what is the most efficient way to get through all of your video files and figure out which ones to add and which ones you want to kick to the trash.  At first it can be very intimidating partly because you may have anywhere from 50 to a few hundred videos you need to sort through.  Hopefully following this format will help you figure out what to keep and what not to.  

1. Begin by bring in all of your footage to your program of choice(I prefer After Effects for editing instead of premier or FCP).  
2. Now open up a time line and bring all your footage into it and have it all starting at around the 10 second mark.  
3. Now one by one drag the layer over to where your playhead is(somewhere around 3 seconds) and remember what is involved with that project and if there is anything worth revising.  If there is not then delete immediately.  If on the other hand there is some footage worth saving for your show reel find the place in the video that you think looks the best and crop it down to something more manageable(like a few seconds or less).  
4. Now go through all your footage this way and once your made it to the end, save and close.  Come back to the project in a few hours or a day to get some fresh eyes on your reel.
5. When you revisit the reel put all the cropped up footage you now have onto the 10 second time marker again and go back to 3 seconds into the video.  Now drag the first clip over take a quick look and drag it back and rate it 1-10. 10 being yes this is awesome it for sure should be in the reel.  While 1 being why didn’t I delete this before.
6. If you did a good job selecting and deleting files beforehand you should end up with numbers hopefully between 10-4 or 5.  Then put all matching the numbers together sequence all your layers to find out how much total time you have in your current reel with all your footage.

Right now you are at a really good point in creating your reel.  You have been able to noodle out all the crappy files and all that is left is trim out the lower numbers until it is somewhere between a minute to a minute and a half of footage.