Foundation plantings can be so much more than a few shrubs placed around the base of a home. These plantings hide the foundation, soften the lines of a house and visually tie the house to the ground and the surrounding area. Foundation plantings also make houses feel less imposing and more inviting.
The first and most important rule to remember when choosing foundation plants is that they should complement the style and scale of your house. Big houses need equally big plants in large swaths, "boxy" houses should use clusters of differently shaped and sized plants to soften the edges, and the architectural elements of asymmetrical houses should balance with your plantings. No matter the size of your home, all foundation plants should be more than 4 feet wide, with plants that reach 8 feet wide all the better.
Foundation plantings can also be used to hide less attractive elements or accentuate important parts of your home. Trees, shrubs and vines can break-up larger expanses of wall, while homes with large amounts of foundation showing can be masked with massive shrubs. Meanwhile, you can highlight your front entrance by placing slightly larger plants nearby. Two small columnar plants on either side of a door will also work and will break any strong horizontal lines to create a more dynamic visual.
A foundation planting should be considered an extension of your home, and "lines" are an important part of good architecture. For example, a low sheared hedge will emphasize horizonal lines, while columnar trees will accent the strong vertical lines of a tall narrow house nicely. Generally speaking, hard, straight lines will attract more attention, while softer lines demand less attention, so take this into consideration when deciding which parts of your home to hide or showcase.
As with all rules, these are meant to be broken. Use them as a guide to get started!