Almost any plant or flower will do in the role of filler so the objective is to consider the entire planting and aim for proportion and a cohesive look. There is no real formula so I just trust my eyes. You can pretty much tell right off the bat whether something looks weird.
Sometimes I might try to coordinate the colors of the mixed group. For example, I may pick up a foliage plant that has a mix of green, yellow and pink in its leaves (there are certain coleus that have this combination) and that will be my starting point for choosing the remaining elements for the container. Or I might try to vary leaf shapes, for instance, my thriller might be the long, pointy leaves of a Red Cordyline so I may choose fillers and spillers that have more rounded shaped leaves. Or I may consider introducing a different texture such as the fuzzy foliage of a Lamb's Ear plant and combine it with a plant that has smooth, shiny leaves.
Keep in mind that you really want to maximize the space of your container so that might mean choosing several fillers. They don't all have to be different, in fact, it might be best to choose two different plants and just buy multiples of them, that way you won't get too overwhelmed. And once again, my gentle reminder that all plants in the group need to have similar requirements for sunlight and water.
Each season I like to plant things that I've never tried before but there are some fillers that I go back to time and again because they are so reliable:
Coreopsis Moonbeam--delicate yellow flowers on airy foliage. Hardy to zone 4, it returns for me every year and it takes well to dividing and transplanting from one pot to another.
Coleus--I've become a fan despite my friend Barbara's vehement opposition to it. Shown in the photo at the beginning of this post, it comes in so many different varieties with foliage options that can help even the most color-challenged gardener coordinate a great looking container.
Geranium--the great thing about geraniums is you can find them absolutely everywhere, they bloom nonstop and love the sun.
Gaura--light, airy flowers on tall, thin stems. Blooms all season and mine return faithfully every year.
Also consider Butterfly Bush as fillers for large containers, New Guinea Impatiens for continuous bloom in shadier areas, Zinnia and Dahlia for color all season long, Heuchera for dependable perennial foliage, and herbs such as Sage for foliage interest (and for cooking--no pork, turkey or roast should be without it!).
Bookshelf: Container Gardening Books