Here are some suggestions for archival pens to sign your inkjet prints. 

Pens

Archival pens should use pigment based ink so that they resist fade for the longest possible amount of time. Black ink is recommended because black is generally the most stable color.

Using a pen is necessary if you are printing on glossy, satin, luster, or semi-gloss inkjet papers. Most of these papers are resin coated (aka Photobase) which means they are plastic on both sides. Using a pigment based ink such is a necessity, especially if you want to write on the uncoated side of the paper.

Sakura Pigma Micron – Available in a number of line width sizes and colors

Archival Gold CD/DVD Twin Tipped Marker – Solvent-free, non-toxic, and alcohol-free

Pencils

Pencils are the most popular way to sign, date, or otherwise label prints made on inkjet matte or cotton fine art media. Look for "carbon pencils" which allow for smooth writing and darker, richer lines.

One example is the General's Layout No.555 carbon pencil. You can find these at Amazon.com and other art supply stores.