Vaughn you are correct. Heating the developer releases water vapor resulting in an increased concentration. The crystals indicate the solution is beyond saturation at room temp. You can either add water (distilled or deionized) until they dissolve or simply filter them out then add fresh developer which is what I usually do. The actual concetration of the developer is not that critical. It is a good idea to filter out the black grit fairly regularly because it can sometimes stain the paper.

You should also monitor the pH of your PO with litmus paper (a pH meter is more accuracy than you need, and they break too easily). I like to keep my PO around pH 6 by adding small amounts of oxalic acid. Once the developer becomes alkaline (pH 7 or more) it can have a detrimental effect on the print quality. You can also do as Dennis suggests and sprinkle in some oxalic acid once in a while and not worry about the litmus paper.

It ain't rocket science, so don't worry about being too anal about this stuff. Some basic maintenance will keep your developer going for a long, long time.