A threshold is a solid foundation for any magic which is intended to effect the home, or everyone who enters the home. Such spells work based on the principal of the threshold as not being just a barrier, but a portal which must be passed through.
One use for magic of this kind is to ward against a particular person or type of entity. to do this it is useful to use materials and objects known to be harmful or hateful to that entity. (garlic for vampires, iron for fairies, things the person is allergic to, audio tape containing music they hate, etc.) As was mentioned before, placing these things in, on, or near the doorway is an effective method, but this may not always be convenient or practical. In this case, using them in a ritual to charge another object, such as a stone a piece of wood, or an ornament which can be attached less obtrusively is perfectly acceptable.
Another use is to place spells upon those who come and go through the door. Spells to encourage health, prosperity, peacefulness and joy are common choices. for these uses it’s best to use an object or material which is associated with your desired outcome, and place it in or above the door frame. Traditionally horseshoes were hung in this way to pour luck on those who entered. This is another situation where amulets hung in doorway can be useful, particularly if people are willing to touch or handle the objects as they come through.
A slightly sneakier method might be to paint or inscribe a spell into the door handle, so that a person HAS to handle it to enter, or building your spell to be charged and renewed by the act of using the knocker or doorbell, if it’s intended to drive off unwanted guests, or help visitors.
