You fat bastard. When should cross practice start? What should we do? Should I try to hook up with a team and a coach? You have a wealth of experience here - a handful of races! That's tons of experience compared to me, and unlike most people, you're willing to share your ignorance. So help a brother out here!
Dave, you don't need to wait for formal instruction to become a thoroughly mediocre cyclocross racer. Here's my practice routine:
First, find a bike that is manifestly unsuitable for riding in the dirt - something with skinny tires and drop bars. Try in vain to compensate for this design flaw by buying ungodly expensive glue-on tires with knobs the size of infant chipmunk nipples, plus some $375 Assos knickers. If you must crash, you should look good doing so.
Then find a hilly off camber field near some woods. Wait until it has rained for a few days. Ride back and forth across the field, paying careful attention to turn tightly and head back the other way across the field, but only turn in spots where there is copious mud, roots, gravel, or off-camber wet grass. Don't worry it won't hurt - the ground. You, it will hurt plenty.
Occasionally, foray into the woods. When you get to a downed tree, hop off, drag the bike over the dead tree any way you know how. Try to bang your shin on the tree, or trip over it - it helps if the tree lacks bark and is somewhat slimy. Don't worry about form, if you are riding with appropriate intensity, you won't be capable of good form. Not until you've mastered bad form anyhow.
Find a short, sharp, steep and preferably slippery hill to practice your run-ups. Ideally, a cross racer should come into the base of the hill at full steam, smoothly dismount as the bike slows, shoulder the bike, and sprint to the top, maintaining momentum the whole way. As a novice, you should ride too far up the hill so that you come to a complete stop, and then stumble off the bike as it starts to roll backwards and tip over. You can even practice getting your feet stuck in the pedals and falling over while still clipped in, if you want. Practice like you will race, right? Once you're off the bike, slowly push it to the top of the runup, falling down once or twice if the ground is slippery. Remount after once again coming to a complete stop.
Every so often you should tip over for no apparent reason. This will balance out the tipovers that have an actual cause, like mud, skinny tires, roadbike geometry, wet grass and off camber turns. Once in a while, shoot out into the road on your mud-covered tires. Try not to low side. When you ride back into the field, skip the culvert, ride through the ditch. Try not to endo. Repeat.
Do this for roughly 50 minutes.
Once you get deep enough into oxygen debt, you'll notice some imaginary Belgian friends standing out on some hellish little corner of your practice area with cowbells, urging you on. If you need to cross train and the hallucenogenic cowbells are bugging you, just get off the bike and bludgeon the fans with your seat post. Consider the bludgeoning to be an upper body workout. Make sure that in your delirium, you aren't mistaking Peter and Ken for your imaginary friends - both have been known to shout tips and encouragement from the sidelines. (Note: the blood pouring out of your ears may make it difficult to hear the cowbells. Don't worry about it if you can't hear cowbells...imaginary cowbells are overrated.)
When you near your imaginary finishing line, let an imaginary team with a ridiculous name - say, the Delaware Cyclocross Coalition of Delaware - blow by you on their cross bikes, just to provide a realistic training experience. It's okay, you can have a good cry now.
After you're done, sit on the side and drink some Le Chouffe with Peter and Ken, or your imaginary friends, and ask yourself what it is you did, to make you want to punish yourself like that. Then go home, and sign up for Granogue, Worcester, and five other 'cross races you've never heard of before. Tell yourself it's okay... you can quit any time you like. Really.