How do I patent an invention without a prototype?

In the U.S.:

You do not need a prototype to get a patent.  The two types are utility patents and design patents.  You need a utility patent, not a design patent.

You can either do it yourself (less expensive, but you might not do it correctly) or use a patent attorney (more expensive, but might be faster).

You do not need a prototype. However, you need to clearly define how to implement your invention and document that in the patent application. You have to understand, an idea cannot be patent, you need to demonstrate to the US Patent and Trademark Office that there is really a way to implement your idea.

You can do it on your own, pretending you save some money. You can also let a professional to do it, and do it right.

In general, the steps are:
1. You do patent search, not only the US Patent and Trademark office's patent database (www.uspto.gov), but also worldwide database. Because if someone have invented your idea years ago and have it patented in United Kingdom, you cannot file the same in US even though that patent was not filed in US.

2. Review the patent search result, and that helps you to improve your invention and improve the chances of being grant a patent later when you file your patent.

3. Decide where in the world you want to file your patent, patent protection are per country basis, that is, a United Kingdom patent will not get protection in US. So for your invention, when successfully produced and marketed, you will have it made in Asia (for example China), sell to US and Europe. Then you need to file patent in China, US, and European Patent Office (to cover more than one countries).

4. File the first patent application in one of the countries you selected. Once it is filed, you have 12-months period to file to the other countries. That is, for example, you file it in USPTO on Jan 1, 2007, then you spend the 12 months' time to market your invention, towards the end of 2007, you find it the invention will fly, then you spend more money to file patent in China and European Patent Office. These filings has to be done before Dec 31, 2007 and claim priority over your original filed US patent.

After first filing, you are more or less protected in terms of your ownership of the patent in the filing countries, so you are more protected when you show certain portion of your idea to factory or potential buyers. And you are the only person who have the right to file patent in other countries within that 12 months period.

Of course, you are recommended to hire some one who know the trade, that is, patent attorney to work this out.