Alter Self (PHB)
The spell states that you take form of a different creature of the same type. Therefore if you only have a metallic dragon miniature, but you need a chromatic one(which cost significantly more), have your tricky dragon be under the guise of Alter Self and surprise the party, rather than simply placing a copper dragon on the table and asking your players to pretend its red.
Alter self also allows the caster to appear one size category smaller or larger. This trick will not only keep your party guessing why that medium dragon is so tough, but will allow you get more mileage out of your minis.
Enlarge/Reduce Person (PHB)
This spell functions much like the example above, but it is limited to humanoids. “Are those ogres, or enlarged orcs?” You can justify using large and huge sized miniatures without changing the creatures CR.
Shadow Conjuration/ Greater Shadow Conjuration (PHB)
Since the Shadow Conjuration spells produce effects and monsters at a fraction of their power (20 and 40 percent respectively), they can be used at low levels to summon very intimidating forgeries. The spell states that illusions look real until the PC succeeds at a will save; that means you can slap down that Pit Fiend or dragon miniature that you otherwise wouldn’t have a chance to use until the party is a much higher level.
This spells works in much the same manner as Shadow Conjuration, but the creature created will be half the power of the original. The spell states that the monster looks just like the original, to include size. Again, this trick allows a clever spell caster (and DM); place a nasty and seemingly unstoppable monster on the table without going over the CR of the encounter. Watch your players pale when you set the Balor on the table in front of their level ten party and then revel in their surprise after he crumbles into a pile of snow and ice after the battle.
Each of these spells will allow you to pull some interesting tricks at the table and provide excellent story hooks (The “gold” dragon that has been burning down villages, for example). In addition, these spells will allow a DM to better use the minis he has rather than buying new ones for a single encounter.