Don't forget about gloves when you're buying riding gear. Too many riders cruise around without any protection on their hands. This means if they go down they're going to mangle their fingers and palms. What's your first reaction when you go down? To put your hands out. This natural impluse will destroy your hands if you don't have some good gloves on. What's a good pair? Roland Sands Barfly Gloves.
Reflective Materials and Bright Colors
When rain falls, visibility is lessened and not just for you. All those car drivers have a harder time seeing things too. Car drivers already have a hard enough time identifying motorcyclists so when it's raining, make sure you wear some type of bright, reflective material. Most waterproof riding suits will have some kind of reflective material already on them. Keep this handy if you suspect you may be caught in the rain.
Rain Resistant is not Waterproof
There's a lot of jackets and riding pants out there that are water or rain resistant. That doesn't mean you won't get wet when wearing these pieces of gear. If you want to stay dry, you really have to get pieces of gear that are actually waterproof. Water resistant gear is good if it rains for a short period of time or if it's lightly raining or misting. Otherwise, go with some serious waterproof gear.
Full Suit or Several Pieces?
I highly recommend a full body suit. While these can be cumbersome to carry and aren't typically as comfortable as a regular pair of pants and a jacket, they do the best job of actually keeping you dry. Still, if you don't want a full body jumpsuit style piece of rain gear, a good waterproof riding jacket and riding pants will suffice in most situations.