Simply pot them up into a one gallon sized nursery planter, the kind with large drainage holes in the bottom (often left over from buying perennials or small shrubs, or you can often get them really cheap, used, at garden centers or even free). Plant them so that there is less than half an inch of rim sticking above the soil.

Then, bury the entire pot in your planter, with the rim level or just slightly below the soil line, and cover that half inch gap with soil.

Be sure to water the individual small pots for a few weeks, until the roots of the plant grow through into the bulk soil beyond.

In the fall, you simply excavate pot and all. Some pruning of roots that grow through the drainage holes may be necessary. For most things, you also will need to cut back the top to compensate for the loss of roots, and it would also be a good time to treat for insects.

I've used this method for several years, and it works great. The plants aren't restricted in growth, and makes it much easier in the fall with less stress on the plants.

Once I bring them in do I have to water them? Should I wait until they become dormant and the cold dies them back?

Those "spikes" they sell as accent plants are often Dracaena marginata, a common houseplant easy to grow into a little tree.

I like being able to buy something different next year!

I got kind of sick of spikes a few years ago, I've been using various grasses and grass like plants for the same effect. I actually have been using grains like barley and oats to good effect, and it's SO cheap, a tiny handful per pot costs nothing.