Obvious spam users (ones with 0 friends and stock photos) tend to outweigh the number of legitimate friend requests on Facebook. How can one reduce the volume of spam users who friend you without killing the ability for legitimate friends to find you (i.e. blocking from search)?
Keep as much information private as you can, make sure your name is displayed as you would expect someone you know would use as a search and use a picture of yourself that would be easily recognizable. If you are hoping old friends that might not recognize you will find you then allowing people to see your hometown or school would help. You pretty much just have to keep as much private as you can.
Even then it may not help, it will depend on where the spam requests are getting your ID from, If they are getting it from your friends friend lists then there is not much you can do.