Most modern doorknob locks have two fingers. The larger finger holds the door closed while the second (smaller) finger only prevents the first finger from being pressed in when it (the second finger) is pressed in by the catchplate of the door. If you can separate the catch plate and the lock sufficiently far, the second finger will slip out enough to permit the first finger to be slipped.
Simple locks, like desks, can be picked fairly easily. Many desks have simple three or four pin locks of only a few levels, and can be consistently picked by a patient person in a few minutes. A small screwdriver and a paper clip will work wonders in practiced hands.
Find a set of double doors. They are particularly easy to pry apart far enough to slip.
The best material for slips so far is soft sheet copper. It is quite flexible, so it can be worked into jambs easily, and can be pre-bent as needed.
Of course, if they’re flexible enough, credit cards, student IDs, etc., work just fine on locks that have been made slippable if the door jamb is wide enough. Wonderfully subtle, quick, and delightfully effective.
This amazing guide provides information on a variety of simple methods for getting through locked doors and ways to slip locks.