Legal forms, letters, and other documents are progressively migrating to digital media; however, the need for applying a signature to them is often still a requirement. While you can print these forms out, sign them and then fax, scan, or e-mail them after signing, another option is to apply a digitally saved version of your signature.
While one approach to doing this is to scan a document containing your signature and then crop it out and paste it in-line with your form, this requires that you keep the image of your signature readily available, and unless you have used transparencies, the image will mask parts of your form if pasted over it. To overcome this, you can use Apple's Signature feature in its Preview application that ships with OS X.
Recently CNET's Jason Cipriani covered how to set up signatures and use them in PDF documents, so to see how to do this, check out his post.
Preview will not only save your signatures, but will also keep track of when each was created and last used, which can be convenient for preventing yourself from using one variation too much or to remind yourself to update the signature if you find it changes from your current version.
Even with a signature set up in Preview (or multiples to add variation), you will still need to make use of it. While signing a PDF document you have open in Preview is easy enough to do, you cannot directly sign image files like JPEGs or PNGs, or sign Word documents and other similar formats. However, this can be done using a workaround with OS X's built-in PDF management services.
In the event you have a non-PDF document that you would like to sign, you can do so by pressing Command-P to open up the print dialogue box for it, followed by choosing "Open PDF in Preview" from the small PDF menu that is at the bottom of the standard print window. This will convert the document's postscript printout to a PDF and open it in Preview, where you can apply signatures. After signing the document you can print it from Preview or again use the same PDF menu to e-mail or otherwise manage it.
This feature should work as described in Preview; however, if it does not and you find signatures are not being captured or stored properly then you can try removing the Preview application's signatures preferences file, which stores the raw data for each signature you have stored. This file is buried in your user account directory at the following location, which you can easily access by highlighting the following line in its entirety, right-clicking it, and choosing Reveal in the Services contextual submenu.
After removing this file, try relaunching Preview and saving your signatures to it again.