For planar binding, what if you have to accept a "devil's mark" (or a demon's mark/angel's mark/whatever) to maintain control over the creature, which is hard to hide? Maybe even influence, a la binding a vestige.
Or the contract is continually enforced by a magic item created as part of the spell, which you must wear (or for higher-level spells, which must exist). Maybe other people can use the item to hijack the creature, as long as they meet a minimum character level. There could also be, say, a purchasable/artefact magic ring which can be used as a focus for multiple binding spells at once. Or some ability which lets you store the bound creature inside the item when you don't need it, genie-style (possibly including release conditions so it can serve as a guard); this also opens up a potential PrC that specialises in binding Fiends of Possession to empower their gear.
If you lose control of a creature it becomes hostile to you, unless you have some feat or class feature which gives you authority over them.