You don't. They can only be forced once, and only bloom once a year outside. However, continue to water and fertilize them through the winter, and then plant them out in your yard come spring. They may miss one year of bloom, but should return and bloom for you in subsequent years, assuming they are the type that do well outside in your climate. Don't be concerned if the tops of the bulbs turn yellow and die back in a few weeks. This is normal.

Paperwhites are not hardy in the north. The OP seems to be from Minnesota and they are strictly an indoor pleasure there, as here in Mass.

Right. Just throw them away now. They are not hardy in your area at all; in fact, they are rather tender everywhere, but they do well where I live. Once they are grown in water, they rarely come back if ever. If you had grown them in soil, they might retrurn in about two to three years, but in your climate it is not expected that they would survive.

thank you, I will dispose them.
Are there any bulbs that you can force more than once?

Oh, yes. But they need to be built up during the rest of the growing year. Hippeastrum are a good example. However, in these cases you are not really forcing them, but having them bloom in the house. There is a difference. Lilies-of- the-Valley are another example. Paperwhites really are not good for forcing more than once. However, they can be planted outside and in the right conditions and in the right zones, they can flower year after year. This is usually in the south or in certain areas of California. It might take them two years to come into flower again, but they often do. If they were forced in water though, it is doubtful that they would. They are cheap enough that most people throw them out and buy new ones each year. I usually force mine indoors for Christmas in soil. This year I did it in water and did not have very good results as I usally do. I will go back to soil again next year. I am definitely throwing these out soon! I have my Chinese Sacred Lilies in soil. They have not opened yet; but they are known to be slow-starters.

You might be interested in getting a good book on bulbs to browse through and see what you think. There are some good ones out there. Perhaps you could learn about what interests you more that way.