Nicholas, you're right, it's not as easy as some people make it sound. Developing for multiple tier environment basically mean that you can swap out one tier without having to affect any of the other tiers. This can take some time to analyze your situation plus design it. The environment I work in is a 3-tier enviroment and it wasn't easy. I don't care how much experience you have you're always gonna run into a snag.

If you're building custom applications for your clients be sure you do your homework and really determine what it is they need. What they want is not alway what they need.

If you have a canned package and you'd like to make scalable then I would suggest looking into Mere Mortals Framework for VFP. Kevin's framework really allows you the flexibility to make scalable apps.