The bona fide intent may be simply sworn to in the application. However, it is recommended that the intent be bolstered by verifiable documents (corporate minutes, reports, plans, contracts, etc.). It is not yet certain what a court will determine to be a "bona fide" intent. Before the mark will actually receive registration, it still must be used in commerce (i.e. interstate or foreign commerce). Once the application is filed with intent to use, and the intended mark has been approved, published for opposition and indicated as allowed, you will have six (6) months in which to verify actual use and submit specimen of use. This six month period can, under proper circumstances, be extended up to a maximum of two years.
Thus, as you can see, if you are able to engage in long term planning, you will be in a position to establish some priority in a mark before you expend large sums of money for advertising, printing, etc. You will also be able to acquire constructive priority on a mark which you "just plain like", assuming you have a bona fide intent to use it. However, PLEASE NOTE: It is not proper to file an intent to use application if you are simply trying to "tie up" the name and you have no real intention to use the mark. You may, under some circumstances be asked to prove that your intent was bona fide: for example, through Board of Director's minutes showing actions moving toward its use, or through proof of expenses being incurred in preparation of product or packaging bearing the name, or some other reasonable proof.
This ability to file before a mark is used will not eliminate the need for thorough searches and care in choosing a mark, since the filing based on intent to use will not give priority over marks already on file, nor over unfiled marks already in use.