I hate the regular guillotine. I never, ever get it. As a wrestler, I learned my lesson getting caught in it a few times after some sloppy shots, but now, I really rarely am ever caught in it.
There are so many modifications to the guillotine, because the standard one just needs improvement. It's a great self defense move, but against an experienced guy, you probably won't catch him.
Here are some of the best modifications I've seen:
From Murilo and Mauricio Rua:
From a Thai clinch, pop his head under your armpit. Lock on a standing guillotine, but to prevent the "lift" defense, underhook with the arm that's not around his throat. Now, with the arm that's around his neck, put that hand in the pit of your underhooking elbow, and figure-4 your arms. Lift and squeeze. He can't drop down to lift you, and the choke is tighter.
From Kenny Florian:
From a guillotine, standing or on the ground, take the arm that's not around his neck, and cinch your grip up so that the outer forearm of your "free" arm (the one that's not around his neck) is touching his trap (shoulder). The elbow of your "free" arm is now pointing down towards his butt, and the forearm is on his shoulder. The choke is tighter and he can't get closer.
If for some reason, you can't do this, or don't want to, figure-4 your arms like you're doing an RNC. (Without an underhook) This is REALLY tight and you'll get a fast tap.
From Mark Hatmaker and Bas Rutten:
From a sprawl-guillotine position, take the forearm that's around his throat and put it in his cheekbone (you're crossfacing him with your inner forearm). Cross face him so that his face turns AWAY from the arm that's holding his head. He's now facing sideways, and your chest/belly is pressing down on the side of his head. With your "free" arm, post on his shoulder, and grip the wrist of the posting arm with your cross-facing arm. Lift up SLOWLY because his neck will pop right away. It's a strong neck crank.