I've used A&L Western several times and it's easy. Fill up a 1-gallon baggie with soil, box it up, and ship it to their Modesto office. In about 2 weeks they email the analysis.

Take that analysis to a farm supply store (not Home Depot) and tell them the size of the area you're treating, and they'll give you a quote on the amendments needed. Till these in before planting, and you success rate dramatically increases. I recommend a soil test and analysys for any garden, even small ones, as without it you're shooting in the dark and possibly applying amendments that are detrimental to your garden. The sample costs about $45, well worth the trouble you'll save.