I think if you approach your beard the way some ladies approach their eyebrows, you should have good success.

That is, get an eyebrow pencil (and a pencil sharpener) that's a little lighter than your beard color. (Take a lady friend with you to Bartell's and try out a few different colors. If you go too dark, your beard will look super fake.) Using gentle, short strokes, fill in the beard area, starting from the outside and working in. Once you're done, use *clear* mascara (sometimes marketed as brow gel) to keep the hairs in place. You can also use a toothbrush to groom your hairs while the brow gel is still wet.

I don't frequently use brow pencils myself, so I find it handy to have some q-tips and gentle facial lotion or makeup remover to fix stray marks. (Lotion works well as a makeup remover, as long as you don't get it near the eyes.)