Thanks for the PM Kelly, I missed this somehow.
Mara, the general 'rule' for skijoring is the dog should be at least 35 lbs. But there's no real cut-off weight because, like Aaron said, you are helping the dog so you are not a dead weight. Just with a smaller dog you need to 'help' more.
Many people skijor with border collies.
I use an X-back harness, a $15 climbing harness (REI) for me, a skijor line with bungee built in and a carbiner to attach the line to my harness. I used to use the heavy quick releases, but they tend to release when you don't want them too. I've stopped using them. Someday I'll buy a better quick release, like this one:
http://www.skijornow.com/skijornowhome.html. Good overview of quick releases:
http://www.skidogs.ca/index.html?n=SkidogDrivers.EquipmentQuickReleases
The best on-line resources are: skidogs.ca at
http://www.skidogs.ca/index.html?n=GeneralInfo.GettingStartedSkijoring, good information and a message board, the group is based in Ottawa. And sled dog central
http://www.sleddogcentral.com/, which has links for places to buy equipment, information about races, and a chat board.
For books I like 'skijor with your dog' by Hoe-Raitto and Kaynor. Really great book, written by two avid skijorers from Fairbanks. One runs border collies, I think. Also 'ski spot run', but this is less comprehensive, and not as useful for the average pet dog owner, in my opinion. A good read, though, with more information on bikejoring and scootering as well. If I were to buy one it would be 'Skijor with your dog'.
I have both the standard X-back harness (http://www.nooksackracing.com/harnesses.html), and the manmat distance 'shortie' harness:
http://www.howlingdogalaska.com/index.php?page=ldharnessinfo. The manmat harness is supposed to be good for skijoring, but I don't think so. It tends to ride up on the trachea and choke the dog if he's pulling hard. I use manmat for canicross (running with pulling dog), but not skijoring. The manmat works best for Iditarod distance dogs, who, individually, are not pulling super hard, and mainly trot/lope.
There's also this cool harness:
http://store.skijornow.com/merchant2/merchant.mvc?Screen=PROD&Store_Code=SN&Product_Code=1111200&Category_Code=DH, which I'd like to try someday.
Commands to start are fairly simple. Line out, lets go, and whoa. Then left and right (haw and gee). On-by for passing distractions and other dogs. There are more, but those are good starters.
Be sure to keep sessions short and fun. Border collies may not have the best feet for snow running, so a few booties may be good to have on hand. You can buy some at any mushing supply place online pretty cheaply.
Also, it's really helpful if you already know how to ski fairly well. It's not too much fun for the dog if you keep falling. There are many ski areas that offer lessons, and some skijor clubs have ski-only practices as well.
I use skate skis, but classic skis work fine too.
Enjoy!