All prey is not created equal. At the very least, there is the difference between vertebrate and invertebrate prey.

Fish, amphibians, reptiles, birds, and mammals are all included in the diets of the many carnivorous and omnivorous reptilian species. Some are generalists, feeding on many different types of prey, while others are specialists, feeding on a limited number of species of prey. Most reptiles feed in the wild only on living prey, though a few species are carrion eaters.

Research into the nutrient content of different prey animals indicates that there is little difference between them nutritionally when considering healthy, properly fed prey species. Thus, from a nutritional standpoint, converting a reptile from feeding upon one type of prey should be acceptable. Unfortunately, the reptile in question may not be so logical about these things, preferring instead to feed on its usual lizards, frogs, or snakes rather than switching to furry mice.

The size of prey fed to a reptile bears directly on the reptile's ability to catch, swallow and digest the prey. A rule of thumb for lizards is that the prey should be no larger than 2/3 the length of the reptile's head. For snakes, the rule is that prey be no wider at its widest point than the widest part of the snake's body. Feeding prey that is too large may result in regurgitation, injuries from swallowing and regurgitation, seizures, partial paralysis, gut impactions, even death.

One way to attempt to convert a non-rodent eater to feed on rodents is to scent the rodent with the reptile's preferred prey. A living or defrosted frozen lizard or frog (or other preferred food item) may be kept on hand to rub against the killed rodent just before offering it for feeding. This will transfer the scent of the preferred prey to the fur or skin of the rodent. Dangling the rodent from a pair of tongs or hemostats will create the illusion of movement. Combined with the scent, this may entice and trick the reptile into feeding.
 
There are several reasons for feeding prekilled rodent prey. Many reptiles become frightened of live prey, especially if they have been bitten before. With young snakes or lizards, the live prey may just be too active for them. Feeding prekilled eliminates both the fear and the risk of injury.

Do not leave invertebrate prey, especially meal worms, king worms, or crickets, in the enclosure with a reptile without also leaving food for the prey. If the reptile does not eat the invertebrates right away, they will soon get hungry and start feeding on whatever is available, which is usually the reptile. Many reptiles become so severely chewed up and stressed out by their prey that they require veterinary care; such reptiles, like snakes who have been attacked by rodents, can be very difficult to get self-feeding again. Another scenting trick is pithing. This involves piercing the braincase of the killed prey with a pin or nail before offering it to the reptile. Never leave live rodents in an enclosure with the reptile. Too many reptiles have died or been permanently disfigured by rodent attacks.

Something to try before pithing, however, is dipping the prekilled prey into some warm chicken broth. This is especially effective in species whose wild diet includes birds. Canned chicken broth may be poured into ice cube trays and frozen, defrosting cubes as needed. Depending on the size and number of prey you need to dip at each feeding, you can use the trays for regular sized cubes or trays for miniature cubes. (Prominent snake breeders Dave and Tracy Barker discovered the efficacy of chicken broth.)

Some reptiles are sensitive to color, and have definite preferences for prey of certain colors. With rodents, this may mean brown or parti-colored mice rather than white mice (after all, there aren't a lot of white or albino mice in the wild, as they tend to not survive long enough to pass on their color genes). This color preference may extend to insect-eaters as well. Adding powdered Spirulina or alfalfa to the food-and-vitamin mix fed to crickets will turn them green, making them more acceptable to reptiles who typically eat green insects in the wild. (Chameleon keeper Alon Coppens discovered this when he ran out of naturally green insects for his picky Nosy Be C. pardalis.)