Anyone have insight into relocation package negotiations, particularly for an external hire?

How does one negotiate this when the time comes? Are these paid up-front or reimbursed? I am seeing more and more positions of interest that are publishing 'relocation assistance offered' or something to that effect. I just don't feel I can confidently negotiate this benefit without turning the employer off. Personally, I do not mind picking up and leaving for a great job, but the costs of a move can be rather intense. I'd appreciate insight from anyone with experience on relocation packages, or, at least, knows of good literature to read up on, regarding the ins and outs for negotiating a relocation package. Thanks for your time and attention.