LoL. Once the guillotine in the guard is sunk in, there is very little you can do besides muscle out of it.
Basically, you have to focus on avoiding it.
The reason the guard guillotine is catching you by surprise is probably doe to your opponent suddenly pulling you in with his legs. You have to expect this. If your opponent consistently pulls you in, let him. Once your forward up on his body slap on the "can opener" neck crank.
For the standing guillotine you have a little more hope. I found a good demo at
http://www.jiu-jitsu.net/techniques/submission/ .
This is a good counter, there is another option if you don't want to pick up your oppoenent. After you put your arm over the shoulder, POST your lower arm against the knee instead of going behind and cupping it. Your position should be relatively safe now, and you can wrap your leg around your opponents and do a trip. Make sure when you trip that you move to the side of your opponent and go into side countrol. If you go straight forward after the trip you will land in his guard (no good).