Uncle Bob is spot on. The factory setting is pretty firm but works well with about 350 pounds on the bike (two up, usually).
If you're going solo, a simple way to adjust it is to set the suspension at the softest setting and ride on a bumpy stretch of road near your house that you're familiar with. Toss in a few low speed passes over speed bumps if you have access to them. If you don't bottom out, you're fine. If you do, adjust it up a notch and try it slightly faster. If you bottom out, go up another notch. With cruisers it's truly trial and error, but once you have it adjusted right, it makes a world of difference. Oh, and if you load the bike for a trip, don't forget to adjust it to compensate for the addtional load...and also to keep your headlight beam off the rearview mirror of the vehicle in front of you.