PageRank can decrease. Losing good Inbound Links can cause a drop in your PageRank. If you have a lot of links from a given website that were the cause of your PageRank to increase, should that website disappear, or for some other reason the pages with the links on them disappear, your own PageRank can decrease.

It is important to:

1. Spread your Inbound Links sources across many domains - if you don't own/control the website upon which they reside.
2. Always be feeding your Inbound Links.