Always have a method of quick release (paramedic shears, quick release snaps, rescue hook, etc) available. A standard pocket knife is not a good tool for this purpose. More great info on this topic can be found on Frozen Meursault's site!
Paramedic Shears
Easily available online or at your local drugstore.
These have a blunt side that is used under the bondage (against the skin) that helps prevent cutting/poking.
Try yours out on the rope/bondage implement that you're planning to use! They can have a surprisingly difficult time getting through many bondage materials (including rope, especially synthetics) and quality varies wildly.
Rescue Hook
Available online – just search for "Benchmade Rescue Hook."
Multiple styles are available – many people prefer a "5" as they can hang it from a DM kit or even a necklace to have it on hand. See here for an example
These work VERY well cutting material that is under tension (load bearing lines especially). They do not work well with cutting rope that has any slack.
Quick release snaps
Available at Home Depot (or online, of course!)
Don’t use “panic snaps” or “quick release snaps” for load bearing (or potentially load bearing) bondage.
Very handy to use with bondage that ties someone in a laying position to a bed, for example – they will release instantly when the quick release is activated. Keep in mind that if you want your bondage to be more difficult to escape from, you should place these out of your bottom's reach!
Notes on cutting rope
Always remember that your rope is much more replaceable than your bottom!
That said, in many situations, it is not necessary (or even desirable) to cut rope.
For specific notes on cutting rope when someone has fallen from suspension, see this article.
When someone needs to be out of bondage NOW, cutting rope should always be a readily available option. Reasons for this could include:
Onset of sharp, shooting pains in a bound limb
Panic/anxiety
Muscle spasm
Asthma attack
Nausea/vomiting
Factors outside the scene (mother-in-law knocking on the bedroom door! House on fire! You get the idea.)
Remember that moving limbs that have been tightly bound, especially in a restrictive/strenuous position, can cause further injury. Support the limbs as the rope is cut to allow the bottom to SLOWLY move them to a neutral position.
If you buy rope from Twisted Monk, they will replace it in the event that it is cut off in an emergency!