Interestingly I use a little different system than most - although its really GGs system with an added twist.
I have to know the person and their reputation as well as watching their posting on this and a few other boards.
I approach the links as traffic links, not as SEO linking - it seems to have a much better bottom line
Although I will say that trading is definitely what helped me achieve the SE positioning I have - but I never did it from a SEO standpoint so you will find sites that have no "Google PR" listed as well as sites that do quite well in the "perceived PR" department.
I also do a lot of linking to sites where I never ask for a trade - in about 90% of the cases it was due to relativity of their site for my surfers and the others were just sites I liked. It paid off in that I now have links returned from about 50% of them just based on them seeing my surfers coming in and being very "productive???"