personal versus professional letters of reference

It should be easy - professional references should be people that you have had significant interaction with in the past three to five years in business situations. If you happen to be friends with them as well, that's nice, but have them write the letter as a professional reference attesting to your business acumen, professionalism, the high regard you are held in in your field, your financial situation, etc. Once you've got the proper amount of the professional letters together, go on to any other people (whether in your professional world or not) that can write a good personal letter attesting to your sterling deportmant, reliabilty, honesty, kindness, and so forth.