If your Kryptonite has a round key, and it is sticking, get some lock lube from the hardware store. After lubing, the lock will work like new.
If your Kryptonite has a flat key, the "sticking" has to do with an anti-picking security design. When the key goes in, it contacts several rotating plates. Each plate has a rectangular cutout, shaped to accept the key. It is easy to rotate the top plate, or a couple of plates, by twisting the key as you pull the key out of the lock. If a plate is out of alignment with the plate under it, the key may get stuck, or the lock may refuse to open or close.
The solution is easier and faster to do in practice than it is to describe. You need to insert the key just about 1/16th of an inch to engage the top rotation plate, and twist the key to align the rectangle on that plate with the plate beneath it. Then do the next plate, and the next plate. Soon, all of the plates are back in alignment, and the lock works fine. Again, this rotating plate design is NOT a defect. This design makes picking the lock more difficult, and is intended to help increase security.
If one or two plates is just slightly out of line, just gently wiggling the key as you insert it realigns the plates. When you withdraw the key, pull it straight out without turning it.
This rotating plate design is used on both OnGuard and Kryptonite locks with the flat key that were sold during 2004 and 2005. I've seen ads showing a 2006 OnGuard lock with a different looking flat key...I don't know if that "new" flat key means OnGuard has a 2006 flat key design without rotating plates.
If you can get by with a shorter lock, my favorite is the Kryptonite Evolution Mini. It is light, compact, and easy to lock and unlock. It fits around any rear wheel on a road bike, and should fit around the rear wheel of most mountain bikes.