This question would probably be better answered on the IPC Designers Council listserver. While there are some good ideas being posted here, there's a lot to be said for the many years of experience gained by people whose primary job is PCB design.
Do not use software packages auto generated outlines, they take into account nothing related to PCB assembly and rework. Likewise, using Silkscreen and hoping will not work. Find out what your pick and place and rework processes require, use those in developing design rules. Our library carries 3 outlines - 1 silkscreen, one actual MMC size, one placement courtyard. As general rules, our process requires .5mm between components, an extra 1.5mm for components taller than 3mm, an extra 3.2mm around BGAs for inspection. Those are just general guidelines to start with, when placing the board you have to add your knowledge of how a board goes together.
Please, don't take that as derogatory, the PCB designers gain that information by communicating with the types of professionals that populate Technet, but as the question was posed from a design aspect, it might be better answered that way.
I'm a bit grumpy about this. I am the only PCB designer in a RFID company of 3000+ employees. Management has decided that pretty much anyone can design PCBs, they laid off all the other designers. So almost all of our products have been missing market dates by months, even years, not meeting performance specs, manufacturing costs are increasing, and field failures have gone up. I spend all of my time correcting problems, not designing anything. Just minutes ago, I viewed CEO message lamenting our 4th missed quarter in a row, the ONLY answer is more workforce reductions, I believe there's a good chance I am viewed as expendable, since "we have other people who can design boards". And another good company bites the dust from mismanagement.
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