Soil!

Soil is perhaps the most important factor in successful gardening, from what I understand. But what is soil? Dirt from the yard? Broken down compost? Potting soil from the garden center? One book I'm reading (Gardening for Geeks by Christy Wilhelmi) states that potting soil isn't soil at all, but a combination of compost (50%), peat or coir (coconut hulls - 40%), and perlite (10%). The peat or coir hold in moisture and nutrients and perlite just takes up space to keep things light and aerated. So here's my plan to fill my beds.

We are lucky enough in our town that the town offers free compost (!) and since we have a pickup, my plan is to take advantage of living in the town in which I do and bring home truck loads full. Once the plan is in place for the beds (size, quantity, etc.) I can figure out exactly how many loads I'll need. 

The tricky part will be the coir (which I can probably reduce the amount a bit if I mix in some of our own clay soil, which itself is good at retaining moisture and nutrients). So I'll need to scout out sources. And perlite, which I'm guessing is fairly easy to come by at the local garden center.

Any tricks for great soil?