The best options are mentioned here. Use shortbow or binoculars to aim. Both available within 5 minutes of tutorial completion. Aim with either, then switch to crossbow, the bolts will aim to the crosshairs. This is how you take out the red drakes tail with a crossbow and 10 or less dexterity. .
Dark Souls Stat and Weapon Calcs http://sites.google.com/site/darksoulstats/
but if you're talking about manual aim, its not possible.
Actually, manual aim is quite possible. The method for manual aim with corssbows requires one key item, the binoculars.
Manual Aiming Steps: 1) Make sure the weapon that you want to aim with and the binoculars are hotkeyed 2) Use binoculars and zoom into the target 3) Center the target in the middle of the screen, both vertically and horrizontally 4) Without exiting the binoculars, click the button to fire the weapon
Note that this works with will spells, miracles, and pyros as well. It is an effective way to "snipe" with lightning spears.